Introduce new hook `ThemeLoader`

library/Icinga/Application/Hook/ThemeLoaderHook.php

@@ -0,0 +1,22 @@
+<?php
+/* Icinga Web 2 | (c) 2022 Icinga GmbH | GPLv2+ */
+
+namespace Icinga\Application\Hook;
+
+/**
+ * Provide an implementation of this hook to dynamically provide themes.
+ * Note that only the first registered hook is utilized. Also note that
+ * for ordinary themes this hook is not required. Place such in your
+ * module's theme path: <module-path>/public/css/themes
+ */
+abstract class ThemeLoaderHook
+{
+    /**
+     * Get the path for the given theme
+     *
+     * @param ?string $theme
+     *
+     * @return ?string The path or NULL if the theme is unknown
+     */
+    abstract public function getThemeFile(?string $theme): ?string;
+}

library/Icinga/Web/StyleSheet.php

@@ -290,6 +290,19 @@ class StyleSheet
         $app = Icinga::app();
 
         if ($theme && $theme !== self::DEFAULT_THEME) {
+            if (Hook::has('ThemeLoader')) {
+                try {
+                    $path = Hook::first('ThemeLoader')->getThemeFile($theme);
+                } catch (Exception $e) {
+                    Logger::error('Failed to call ThemeLoader hook: %s', $e);
+                    $path = null;
+                }
+
+                if ($path !== null) {
+                    return $path;
+                }
+            }
+
             if (($pos = strpos($theme, '/')) !== false) {
                 $moduleName = substr($theme, 0, $pos);
                 $theme = substr($theme, $pos + 1);