feat: vanilla CSS color scheme changes (#3996)

* feat: vanilla CSS color scheme changes
* chore: scheme mixin
* chore: remove darkmode & colored header less variables
* feat: high contrast schemes

framework/core/migrations/2024_06_07_000000_change_to_theme_mode_setting_and_preference.php

@@ -0,0 +1,56 @@
+<?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.
+ */
+
+use Illuminate\Database\Schema\Builder;
+
+return [
+    'up' => function (Builder $schema) {
+        $darkMode = $schema->getConnection()
+            ->table('settings')
+            ->where('key', 'theme_dark_mode')
+            ->first();
+
+        $schema->getConnection()
+            ->table('settings')
+            ->insert([
+                [
+                    'key' => 'color_scheme',
+                    'value' => $darkMode === '1' ? 'dark' : 'auto',
+                ],
+                [
+                    'key' => 'allow_user_color_scheme',
+                    'value' => '1',
+                ]
+            ]);
+
+        $schema->getConnection()
+            ->table('settings')
+            ->where('key', 'theme_dark_mode')
+            ->delete();
+    },
+
+    'down' => function (Builder $schema) {
+        $themeMode = $schema->getConnection()
+            ->table('settings')
+            ->where('key', 'color_scheme')
+            ->first();
+
+        $schema->getConnection()
+            ->table('settings')
+            ->insert([
+                'key' => 'theme_dark_mode',
+                'value' => $themeMode === 'dark' ? '1' : '0',
+            ]);
+
+        $schema->getConnection()
+            ->table('settings')
+            ->whereIn('key', ['color_scheme', 'allow_user_color_scheme'])
+            ->delete();
+    }
+];