diff --git a/.editorconfig b/.editorconfig
index 0fb398499..e81de0fe0 100644
--- a/.editorconfig
+++ b/.editorconfig
@@ -18,7 +18,7 @@ trim_trailing_whitespace = false
 [*.{php,xml,json}]
 indent_size = 4
 
-[{tsconfig.json,prettierrc.json}]
+[{tsconfig.json,prettierrc.json,package.json}]
 indent_size = 2
 
 [*.neon]
diff --git a/framework/core/js/src/admin/components/EditCustomCssModal.tsx b/framework/core/js/src/admin/components/EditCustomCssModal.tsx
index 432961074..6c71e4172 100644
--- a/framework/core/js/src/admin/components/EditCustomCssModal.tsx
+++ b/framework/core/js/src/admin/components/EditCustomCssModal.tsx
@@ -1,7 +1,19 @@
 import app from '../../admin/app';
-import SettingsModal from './SettingsModal';
+import SettingsModal, { type ISettingsModalAttrs } from './SettingsModal';
+import Mithril from 'mithril';
 
 export default class EditCustomCssModal extends SettingsModal {
+  oninit(vnode: Mithril.Vnode<ISettingsModalAttrs, this>) {
+    super.oninit(vnode);
+
+    if (this.setting('custom_less_error')()) {
+      this.alertAttrs = {
+        type: 'error',
+        content: this.setting('custom_less_error')(),
+      };
+    }
+  }
+
   className() {
     return 'EditCustomCssModal TextareaCodeModal Modal--large';
   }
diff --git a/framework/core/src/Forum/ForumServiceProvider.php b/framework/core/src/Forum/ForumServiceProvider.php
index dff421987..e8c05217a 100644
--- a/framework/core/src/Forum/ForumServiceProvider.php
+++ b/framework/core/src/Forum/ForumServiceProvider.php
@@ -133,7 +133,7 @@ class ForumServiceProvider extends AbstractServiceProvider
                 $sources->addFile(__DIR__.'/../../less/forum.less');
                 $sources->addString(function () use ($container) {
                     return $container->make(SettingsRepositoryInterface::class)->get('custom_less', '');
-                });
+                }, 'custom_less');
             });
 
             $container->make(AddTranslations::class)->forFrontend('forum')->to($assets);
@@ -195,7 +195,8 @@ class ForumServiceProvider extends AbstractServiceProvider
                     $container->make('flarum.assets.forum'),
                     $container->make('flarum.locales'),
                     $container,
-                    $container->make('flarum.less.config')
+                    $container->make(SettingsRepositoryInterface::class),
+                    $container->make('flarum.less.config'),
                 );
                 $validator->whenSettingsSaved($event);
             }
@@ -208,7 +209,8 @@ class ForumServiceProvider extends AbstractServiceProvider
                     $container->make('flarum.assets.forum'),
                     $container->make('flarum.locales'),
                     $container,
-                    $container->make('flarum.less.config')
+                    $container->make(SettingsRepositoryInterface::class),
+                    $container->make('flarum.less.config'),
                 );
                 $validator->whenSettingsSaving($event);
             }
diff --git a/framework/core/src/Forum/ValidateCustomLess.php b/framework/core/src/Forum/ValidateCustomLess.php
index e397a13d9..0ed456f68 100644
--- a/framework/core/src/Forum/ValidateCustomLess.php
+++ b/framework/core/src/Forum/ValidateCustomLess.php
@@ -32,7 +32,8 @@ class ValidateCustomLess
         protected Assets $assets,
         protected LocaleManager $locales,
         protected Container $container,
-        protected array $customLessSettings = []
+        protected SettingsRepositoryInterface $settings,
+        protected array $customLessSettings = [],
     ) {
     }
 
@@ -72,6 +73,8 @@ class ValidateCustomLess
         $adapter = new InMemoryFilesystemAdapter();
         $this->assets->setAssetsDir(new FilesystemAdapter(new Filesystem($adapter), $adapter));
 
+        $this->settings->delete('custom_less_error');
+
         try {
             $this->assets->makeCss()->commit();
 
@@ -82,6 +85,10 @@ class ValidateCustomLess
             throw new ValidationException(['custom_less' => $e->getMessage()]);
         }
 
+        if (! empty($this->settings->get('custom_less_error'))) {
+            throw new ValidationException(['custom_less' => $this->settings->get('custom_less_error')]);
+        }
+
         $this->assets->setAssetsDir($assetsDir);
         $this->container->instance(SettingsRepositoryInterface::class, $settings);
     }
diff --git a/framework/core/src/Frontend/Compiler/LessCompiler.php b/framework/core/src/Frontend/Compiler/LessCompiler.php
index 734e4c59c..0644ccc00 100644
--- a/framework/core/src/Frontend/Compiler/LessCompiler.php
+++ b/framework/core/src/Frontend/Compiler/LessCompiler.php
@@ -12,6 +12,7 @@ namespace Flarum\Frontend\Compiler;
 use Flarum\Frontend\Compiler\Source\FileSource;
 use Illuminate\Support\Collection;
 use Illuminate\Support\Str;
+use Less_Exception_Compiler;
 use Less_FileManager;
 use Less_Parser;
 use Less_Tree_Import;
@@ -71,6 +72,10 @@ class LessCompiler extends RevisionCompiler
             return '';
         }
 
+        if (! empty($this->settings->get('custom_less_error'))) {
+            unset($sources['custom_less']);
+        }
+
         ini_set('xdebug.max_nesting_level', '200');
 
         $parser = new Less_Parser([
@@ -96,7 +101,27 @@ class LessCompiler extends RevisionCompiler
             $parser->registerFunction($name, $callback);
         }
 
-        return $this->finalize($parser->getCss());
+        try {
+            $compiled = $this->finalize($parser->getCss());
+
+            if (isset($sources['custom_less'])) {
+                $this->settings->delete('custom_less_error');
+            }
+
+            return $compiled;
+        } catch (Less_Exception_Compiler $e) {
+            if (isset($sources['custom_less'])) {
+                unset($sources['custom_less']);
+
+                $compiled = $this->compile($sources);
+
+                $this->settings->set('custom_less_error', $e->getMessage());
+
+                return $compiled;
+            }
+
+            throw $e;
+        }
     }
 
     protected function finalize(string $parsedCss): string
diff --git a/framework/core/src/Frontend/Compiler/RevisionCompiler.php b/framework/core/src/Frontend/Compiler/RevisionCompiler.php
index e47ef9bd7..4cd5aad5f 100644
--- a/framework/core/src/Frontend/Compiler/RevisionCompiler.php
+++ b/framework/core/src/Frontend/Compiler/RevisionCompiler.php
@@ -13,6 +13,7 @@ use Flarum\Frontend\Compiler\Concerns\HasSources;
 use Flarum\Frontend\Compiler\Source\FileSource;
 use Flarum\Frontend\Compiler\Source\SourceInterface;
 use Flarum\Frontend\Compiler\Source\StringSource;
+use Flarum\Settings\SettingsRepositoryInterface;
 use Illuminate\Contracts\Filesystem\Cloud;
 
 /**
@@ -29,6 +30,7 @@ class RevisionCompiler implements CompilerInterface
     public function __construct(
         protected Cloud $assetsDir,
         protected string $filename,
+        protected SettingsRepositoryInterface $settings
     ) {
         $this->versioner = new FileVersioner($assetsDir);
     }
diff --git a/framework/core/src/Frontend/Compiler/Source/SourceCollector.php b/framework/core/src/Frontend/Compiler/Source/SourceCollector.php
index 66cb52be4..4bd1e0435 100644
--- a/framework/core/src/Frontend/Compiler/Source/SourceCollector.php
+++ b/framework/core/src/Frontend/Compiler/Source/SourceCollector.php
@@ -35,12 +35,18 @@ class SourceCollector
         return $this;
     }
 
-    public function addString(Closure $callback): static
+    public function addString(Closure $callback, ?string $key = null): static
     {
-        $this->sources[] = $this->validateSourceType(
+        $source = $this->validateSourceType(
             new StringSource($callback)
         );
 
+        if (! empty($key)) {
+            $this->sources[$key] = $source;
+        } else {
+            $this->sources[] = $source;
+        }
+
         return $this;
     }