diff --git a/framework/core/src/Extend/Theme.php b/framework/core/src/Extend/Theme.php
index 26a860c4e..0d332b862 100644
--- a/framework/core/src/Extend/Theme.php
+++ b/framework/core/src/Extend/Theme.php
@@ -19,6 +19,7 @@ class Theme implements ExtenderInterface
     private $lessImportOverrides = [];
     private $fileSourceOverrides = [];
     private $customFunctions = [];
+    private $lessVariables = [];
 
     /**
      * This can be used to override LESS files that are imported within the code.
@@ -100,12 +101,51 @@ class Theme implements ExtenderInterface
         return $this;
     }
 
+    /**
+     * Defines a new Less variable to be accessible in all Less files.
+     *
+     * If you want to expose a setting from your database to Less, you should use
+     * the `registerLessConfigVar` extender from `Extend\Settings` instead.
+     *
+     * Please note the value returned from the callable will be inserted directly
+     * into the Less source. If it is unsafe in some way (e.g., contains a
+     * semi-colon), this will result in potential security issues with your
+     * stylesheet.
+     *
+     * Likewise, if you need your variable to be a string, you should surround it
+     * with quotes yourself.
+     *
+     * ```php
+     * (new Extend\Theme())
+     *   ->addCustomLessVariable('my-extension__asset_path', function () {
+     *     $url = resolve(UrlGenerator::class);
+     *     $assetUrl = $url->to('forum')->base().'/assets/extensions/my-extension/my-asset.jpg';
+     *     return "\"$assetUrl\"";
+     *   })
+     * ```
+     *
+     * @param string $variableName Name of the variable identifier.
+     * @param callable $value The PHP function to run, which returns the value for the variable.
+     *
+     * @return self
+     */
+    public function addCustomLessVariable(string $variableName, callable $value): self
+    {
+        $this->lessVariables[$variableName] = $value;
+
+        return $this;
+    }
+
     public function extend(Container $container, Extension $extension = null)
     {
         $container->extend('flarum.frontend.custom_less_functions', function (array $customFunctions) {
             return array_merge($customFunctions, $this->customFunctions);
         });
 
+        $container->extend('flarum.less.custom_variables', function (array $lessVariables) {
+            return array_merge($this->lessVariables, $lessVariables);
+        });
+
         $container->extend('flarum.assets.factory', function (callable $factory) {
             return function (...$args) use ($factory) {
                 /** @var Assets $assets */
diff --git a/framework/core/src/Frontend/FrontendServiceProvider.php b/framework/core/src/Frontend/FrontendServiceProvider.php
index 86af8f1bb..bcb5a11bd 100644
--- a/framework/core/src/Frontend/FrontendServiceProvider.php
+++ b/framework/core/src/Frontend/FrontendServiceProvider.php
@@ -156,6 +156,13 @@ class FrontendServiceProvider extends AbstractServiceProvider
                 ],
             ];
         });
+
+        $this->container->singleton(
+            'flarum.less.custom_variables',
+            function (Container $container) {
+                return [];
+            }
+        );
     }
 
     /**
@@ -184,9 +191,11 @@ class FrontendServiceProvider extends AbstractServiceProvider
     {
         $sources->addString(function () {
             $vars = $this->container->make('flarum.less.config');
+            $extDefinedVars = $this->container->make('flarum.less.custom_variables');
+
             $settings = $this->container->make(SettingsRepositoryInterface::class);
 
-            return array_reduce(array_keys($vars), function ($string, $name) use ($vars, $settings) {
+            $customLess = array_reduce(array_keys($vars), function ($string, $name) use ($vars, $settings) {
                 $var = $vars[$name];
                 $value = $settings->get($var['key'], $var['default'] ?? null);
 
@@ -196,6 +205,12 @@ class FrontendServiceProvider extends AbstractServiceProvider
 
                 return $string."@$name: {$value};";
             }, '');
+
+            foreach ($extDefinedVars as $name => $value) {
+                $customLess .= "@$name: {$value()};";
+            }
+
+            return $customLess;
         });
     }
 }
diff --git a/framework/core/tests/fixtures/less/custom_variable.less b/framework/core/tests/fixtures/less/custom_variable.less
new file mode 100644
index 000000000..a8db66edc
--- /dev/null
+++ b/framework/core/tests/fixtures/less/custom_variable.less
@@ -0,0 +1,3 @@
+.dummy_var_test {
+  --x: @doesnt-exist;
+}
diff --git a/framework/core/tests/integration/extenders/ThemeTest.php b/framework/core/tests/integration/extenders/ThemeTest.php
index ba8e50e0b..57faea58f 100644
--- a/framework/core/tests/integration/extenders/ThemeTest.php
+++ b/framework/core/tests/integration/extenders/ThemeTest.php
@@ -132,4 +132,28 @@ class ThemeTest extends TestCase
         $this->assertStringContainsString('.dummy_func_test{color:green}', $contents);
         $this->assertStringContainsString('.dummy_func_test2{--x:1000;--y:false}', $contents);
     }
+
+    /**
+     * @test
+     */
+    public function theme_extender_can_add_custom_variable()
+    {
+        $this->extend(
+            (new Extend\Frontend('forum'))
+                ->css(__DIR__.'/../../fixtures/less/custom_variable.less'),
+            (new Extend\Theme)
+                ->addCustomLessVariable('doesnt-exist', function () {
+                    return 'it does';
+                })
+        );
+
+        $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->assertStringContainsString('.dummy_var_test{--x:it does}', $contents);
+    }
 }