Merge pull request #53921 from nextcloud/backport/53857/stable31

[stable31] fix(theming): Correctly generate CSS for font themes

Author: nickvergessen

apps/theming/lib/Controller/ThemingController.php

@@ -401,7 +401,17 @@ public function getThemeStylesheet(string $themeId, bool $plain = false, bool $w
 			$css = ":root { $variables } " . $customCss;
 		} else {
 			// If not set, we'll rely on the body class
-			$css = "[data-theme-$themeId] { $variables $customCss }";
+			// We need to separate @-rules from normal selectors, as they can't be nested
+			// This is a replacement for the SCSS compiler that did this automatically before f1448fcf0777db7d4254cb0a3ef94d63be9f7a24
+			// We need a better way to handle this, but for now we just remove comments and split the at-rules
+			// from the rest of the CSS.
+			$customCssWithoutComments = preg_replace('!/\*.*?\*/!s', '', $customCss);
+			$customCssWithoutComments = preg_replace('!//.*!', '', $customCssWithoutComments);
+			preg_match_all('/(@[^{]+{(?:[^{}]*|(?R))*})/', $customCssWithoutComments, $atRules);
+			$atRulesCss = implode('', $atRules[0]);
+			$scopedCss = preg_replace('/(@[^{]+{(?:[^{}]*|(?R))*})/', '', $customCssWithoutComments);
+
+			$css = "$atRulesCss [data-theme-$themeId] { $variables $scopedCss }";
 		}
 
 		try {