diff --git a/framework/core/src/Extend/Frontend.php b/framework/core/src/Extend/Frontend.php
index 0cf2938e3..9484a1213 100644
--- a/framework/core/src/Extend/Frontend.php
+++ b/framework/core/src/Extend/Frontend.php
@@ -153,9 +153,9 @@ class Frontend implements ExtenderInterface
             }
 
             if ($this->css) {
-                $assets->css(function (SourceCollector $sources) {
+                $assets->css(function (SourceCollector $sources) use ($moduleName) {
                     foreach ($this->css as $path) {
-                        $sources->addFile($path);
+                        $sources->addFile($path, $moduleName);
                     }
                 });
             }
diff --git a/framework/core/src/Extend/Theme.php b/framework/core/src/Extend/Theme.php
new file mode 100644
index 000000000..74fc4fa0e
--- /dev/null
+++ b/framework/core/src/Extend/Theme.php
@@ -0,0 +1,69 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * For detailed copyright and license information, please view the
+ * LICENSE file that was distributed with this source code.
+ */
+
+namespace Flarum\Extend;
+
+use Flarum\Extension\Extension;
+use Flarum\Frontend\Assets;
+use Illuminate\Contracts\Container\Container;
+
+class Theme implements ExtenderInterface
+{
+    private $lessImportOverrides = [];
+    private $fileSourceOverrides = [];
+
+    /**
+     * This can be used to override LESS files that are imported within the code.
+     * For example, core's `forum.less` file imports a `forum/DiscussionListItem.less` file.
+     * The contents of this file can be overriden with this method.
+     *
+     * @param string $file : Relative path of the file to override, for example: `forum/Hero.less`
+     * @param string $newFilePath : Absolute path of the new file.
+     * @param string|null $extensionId : If overriding an extension file, specify its ID, for example: `flarum-tags`.
+     * @return self
+     */
+    public function overrideLessImport(string $file, string $newFilePath, string $extensionId = null): self
+    {
+        $this->lessImportOverrides[] = compact('file', 'newFilePath', 'extensionId');
+
+        return $this;
+    }
+
+    /**
+     * This method allows overriding LESS file sources.
+     * For example `forum.less`, `admin.less`, `mixins.less` and `variables.less` are file sources,
+     * and can therefore be overriden using this method.
+     *
+     * @param string $file : Name of the file to override, for example: `admin.less`
+     * @param string $newFilePath : Absolute path of the new file.
+     * @param string|null $extensionId : If overriding an extension file, specify its ID, for example: `flarum-tags`.
+     * @return self
+     */
+    public function overrideFileSource(string $file, string $newFilePath, string $extensionId = null): self
+    {
+        $this->fileSourceOverrides[] = compact('file', 'newFilePath', 'extensionId');
+
+        return $this;
+    }
+
+    public function extend(Container $container, Extension $extension = null)
+    {
+        $container->extend('flarum.assets.factory', function (callable $factory) {
+            return function (...$args) use ($factory) {
+                /** @var Assets $assets */
+                $assets = $factory(...$args);
+
+                $assets->addLessImportOverrides($this->lessImportOverrides);
+                $assets->addFileSourceOverrides($this->fileSourceOverrides);
+
+                return $assets;
+            };
+        });
+    }
+}
diff --git a/framework/core/src/Frontend/Assets.php b/framework/core/src/Frontend/Assets.php
index 8b793697e..d52ecdfa4 100644
--- a/framework/core/src/Frontend/Assets.php
+++ b/framework/core/src/Frontend/Assets.php
@@ -52,6 +52,16 @@ class Assets
      */
     protected $lessImportDirs;
 
+    /**
+     * @var array
+     */
+    protected $lessImportOverrides = [];
+
+    /**
+     * @var array
+     */
+    protected $fileSourceOverrides = [];
+
     public function __construct(string $name, Filesystem $assetsDir, string $cacheDir = null, array $lessImportDirs = null)
     {
         $this->name = $name;
@@ -155,6 +165,14 @@ class Assets
             $compiler->setImportDirs($this->lessImportDirs);
         }
 
+        if ($this->lessImportOverrides) {
+            $compiler->setLessImportOverrides($this->lessImportOverrides);
+        }
+
+        if ($this->fileSourceOverrides) {
+            $compiler->setFileSourceOverrides($this->fileSourceOverrides);
+        }
+
         return $compiler;
     }
 
@@ -197,4 +215,14 @@ class Assets
     {
         $this->lessImportDirs = $lessImportDirs;
     }
+
+    public function addLessImportOverrides(array $lessImportOverrides)
+    {
+        $this->lessImportOverrides = array_merge($this->lessImportOverrides, $lessImportOverrides);
+    }
+
+    public function addFileSourceOverrides(array $fileSourceOverrides)
+    {
+        $this->fileSourceOverrides = array_merge($this->fileSourceOverrides, $fileSourceOverrides);
+    }
 }
diff --git a/framework/core/src/Frontend/Compiler/LessCompiler.php b/framework/core/src/Frontend/Compiler/LessCompiler.php
index 615c23f22..a88061624 100644
--- a/framework/core/src/Frontend/Compiler/LessCompiler.php
+++ b/framework/core/src/Frontend/Compiler/LessCompiler.php
@@ -10,6 +10,8 @@
 namespace Flarum\Frontend\Compiler;
 
 use Flarum\Frontend\Compiler\Source\FileSource;
+use Illuminate\Support\Collection;
+use Illuminate\Support\Str;
 use Less_Parser;
 
 /**
@@ -27,6 +29,16 @@ class LessCompiler extends RevisionCompiler
      */
     protected $importDirs = [];
 
+    /**
+     * @var Collection
+     */
+    protected $lessImportOverrides;
+
+    /**
+     * @var Collection
+     */
+    protected $fileSourceOverrides;
+
     public function getCacheDir(): string
     {
         return $this->cacheDir;
@@ -47,6 +59,16 @@ class LessCompiler extends RevisionCompiler
         $this->importDirs = $importDirs;
     }
 
+    public function setLessImportOverrides(array $lessImportOverrides)
+    {
+        $this->lessImportOverrides = new Collection($lessImportOverrides);
+    }
+
+    public function setFileSourceOverrides(array $fileSourceOverrides)
+    {
+        $this->fileSourceOverrides = new Collection($fileSourceOverrides);
+    }
+
     /**
      * @throws \Less_Exception_Parser
      */
@@ -61,9 +83,14 @@ class LessCompiler extends RevisionCompiler
         $parser = new Less_Parser([
             'compress' => true,
             'cache_dir' => $this->cacheDir,
-            'import_dirs' => $this->importDirs
+            'import_dirs' => $this->importDirs,
+            'import_callback' => $this->lessImportOverrides ? $this->overrideImports($sources) : null,
         ]);
 
+        if ($this->fileSourceOverrides) {
+            $sources = $this->overrideSources($sources);
+        }
+
         foreach ($sources as $source) {
             if ($source instanceof FileSource) {
                 $parser->parseFile($source->getPath());
@@ -75,6 +102,54 @@ class LessCompiler extends RevisionCompiler
         return $parser->getCss();
     }
 
+    protected function overrideSources(array $sources): array
+    {
+        foreach ($sources as $source) {
+            if ($source instanceof FileSource) {
+                $basename = basename($source->getPath());
+                $override = $this->fileSourceOverrides
+                    ->where('file', $basename)
+                    ->firstWhere('extensionId', $source->getExtensionId());
+
+                if ($override) {
+                    $source->setPath($override['newFilePath']);
+                }
+            }
+        }
+
+        return $sources;
+    }
+
+    protected function overrideImports(array $sources): callable
+    {
+        $baseSources = (new Collection($sources))->filter(function ($source) {
+            return $source instanceof Source\FileSource;
+        })->map(function (FileSource $source) {
+            $path = realpath($source->getPath());
+            $path = Str::beforeLast($path, '/less/');
+
+            return [
+                'path' => $path,
+                'extensionId' => $source->getExtensionId(),
+            ];
+        })->unique('path');
+
+        return function ($evald) use ($baseSources): ?array {
+            $relativeImportPath = Str::of($evald->PathAndUri()[0])->split('/\/less\//');
+            $extensionId = $baseSources->where('path', $relativeImportPath->first())->pluck('extensionId')->first();
+
+            $overrideImport = $this->lessImportOverrides
+                ->where('file', $relativeImportPath->last())
+                ->firstWhere('extensionId', $extensionId);
+
+            if (! $overrideImport) {
+                return null;
+            }
+
+            return [$overrideImport['newFilePath'], $evald->PathAndUri()[1]];
+        };
+    }
+
     protected function getCacheDifferentiator(): ?array
     {
         return [
diff --git a/framework/core/src/Frontend/Compiler/Source/FileSource.php b/framework/core/src/Frontend/Compiler/Source/FileSource.php
index f85c5926e..6fcd9b642 100644
--- a/framework/core/src/Frontend/Compiler/Source/FileSource.php
+++ b/framework/core/src/Frontend/Compiler/Source/FileSource.php
@@ -21,16 +21,22 @@ class FileSource implements SourceInterface
      */
     protected $path;
 
+    /**
+     * @var string
+     */
+    protected $extensionId;
+
     /**
      * @param string $path
      */
-    public function __construct(string $path)
+    public function __construct(string $path, ?string $extensionId = null)
     {
         if (! file_exists($path)) {
             throw new InvalidArgumentException("File not found at path: $path");
         }
 
         $this->path = $path;
+        $this->extensionId = $extensionId;
     }
 
     /**
@@ -56,4 +62,14 @@ class FileSource implements SourceInterface
     {
         return $this->path;
     }
+
+    public function setPath(string $path): void
+    {
+        $this->path = $path;
+    }
+
+    public function getExtensionId(): ?string
+    {
+        return $this->extensionId;
+    }
 }
diff --git a/framework/core/src/Frontend/Compiler/Source/SourceCollector.php b/framework/core/src/Frontend/Compiler/Source/SourceCollector.php
index ec537b26b..452d78e87 100644
--- a/framework/core/src/Frontend/Compiler/Source/SourceCollector.php
+++ b/framework/core/src/Frontend/Compiler/Source/SourceCollector.php
@@ -23,9 +23,9 @@ class SourceCollector
      * @param string $file
      * @return $this
      */
-    public function addFile(string $file)
+    public function addFile(string $file, string $extensionId = null)
     {
-        $this->sources[] = new FileSource($file);
+        $this->sources[] = new FileSource($file, $extensionId);
 
         return $this;
     }
diff --git a/framework/core/tests/fixtures/less/Imported.less b/framework/core/tests/fixtures/less/Imported.less
new file mode 100644
index 000000000..b6cdd664b
--- /dev/null
+++ b/framework/core/tests/fixtures/less/Imported.less
@@ -0,0 +1,3 @@
+.Imported {
+  // ...
+}
diff --git a/framework/core/tests/fixtures/less/dummy.less b/framework/core/tests/fixtures/less/dummy.less
new file mode 100644
index 000000000..e882088e6
--- /dev/null
+++ b/framework/core/tests/fixtures/less/dummy.less
@@ -0,0 +1 @@
+.dummy_test_case{color:red}
diff --git a/framework/core/tests/fixtures/less/forum.less b/framework/core/tests/fixtures/less/forum.less
new file mode 100644
index 000000000..ab882c025
--- /dev/null
+++ b/framework/core/tests/fixtures/less/forum.less
@@ -0,0 +1,5 @@
+@import 'Imported';
+
+.dummy {
+  color: yellow;
+}
diff --git a/framework/core/tests/fixtures/less/override_filesource.less b/framework/core/tests/fixtures/less/override_filesource.less
new file mode 100644
index 000000000..7f69ee733
--- /dev/null
+++ b/framework/core/tests/fixtures/less/override_filesource.less
@@ -0,0 +1,3 @@
+body {
+  color: orange;
+}
diff --git a/framework/core/tests/integration/extenders/ThemeTest.php b/framework/core/tests/integration/extenders/ThemeTest.php
new file mode 100644
index 000000000..87b828494
--- /dev/null
+++ b/framework/core/tests/integration/extenders/ThemeTest.php
@@ -0,0 +1,107 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * For detailed copyright and license information, please view the
+ * LICENSE file that was distributed with this source code.
+ */
+
+namespace Flarum\Tests\integration\extenders;
+
+use Flarum\Extend;
+use Flarum\Testing\integration\TestCase;
+
+class ThemeTest extends TestCase
+{
+    /**
+     * @test
+     */
+    public function theme_extender_override_import_doesnt_work_by_default()
+    {
+        $response = $this->send($this->request('GET', '/'));
+
+        $this->assertEquals(200, $response->getStatusCode());
+
+        $cssFilePath = $this->app()->getContainer()->make('filesystem')->disk('flarum-assets')->path('forum.css');
+        $this->assertStringNotContainsString('.dummy_test_case{color:red}', file_get_contents($cssFilePath));
+    }
+
+    /**
+     * @test
+     */
+    public function theme_extender_override_import_works()
+    {
+        $this->extend(
+            (new Extend\Theme)
+                ->overrideLessImport('forum/Hero.less', __DIR__.'/../../fixtures/less/dummy.less')
+        );
+
+        $response = $this->send($this->request('GET', '/'));
+
+        $this->assertEquals(200, $response->getStatusCode());
+
+        $cssFilePath = $this->app()->getContainer()->make('filesystem')->disk('flarum-assets')->path('forum.css');
+
+        $this->assertStringContainsString('.dummy_test_case{color:red}', file_get_contents($cssFilePath));
+    }
+
+    /**
+     * @test
+     */
+    public function theme_extender_override_import_works_with_external_sources()
+    {
+        $this->extend(
+            (new Extend\Frontend('forum'))
+                ->css(__DIR__.'/../../fixtures/less/forum.less'),
+            (new Extend\Theme)
+                ->overrideLessImport('Imported.less', __DIR__.'/../../fixtures/less/dummy.less', 'site-custom')
+        );
+
+        $response = $this->send($this->request('GET', '/'));
+
+        $this->assertEquals(200, $response->getStatusCode());
+
+        $cssFilePath = $this->app()->getContainer()->make('filesystem')->disk('flarum-assets')->path('forum.css');
+        $contents = file_get_contents($cssFilePath);
+
+        $this->assertStringNotContainsString('.Imported', $contents);
+        $this->assertStringContainsString('.dummy_test_case{color:red}', $contents);
+        $this->assertStringContainsString('.dummy{color:yellow}', $contents);
+    }
+
+    /**
+     * @test
+     */
+    public function theme_extender_override_file_source_works()
+    {
+        $this->extend(
+            (new Extend\Theme)
+                ->overrideFileSource('forum.less', __DIR__.'/../../fixtures/less/override_filesource.less')
+        );
+
+        $response = $this->send($this->request('GET', '/'));
+
+        $this->assertEquals(200, $response->getStatusCode());
+
+        $cssFilePath = $this->app()->getContainer()->make('filesystem')->disk('flarum-assets')->path('forum.css');
+
+        $this->assertEquals('body{color:orange}', file_get_contents($cssFilePath));
+    }
+
+    /**
+     * @test
+     */
+    public function theme_extender_override_file_source_works_by_failing_when_necessary()
+    {
+        $this->extend(
+            (new Extend\Theme)
+                ->overrideFileSource('mixins.less', __DIR__.'/../../fixtures/less/dummy.less')
+        );
+
+        $response = $this->send($this->request('GET', '/'));
+
+        $this->assertStringContainsString('Less_Exception_Compiler', $response->getBody()->getContents());
+        $this->assertEquals(500, $response->getStatusCode());
+    }
+}