Fix: add cache-busting to theme CSS @import

app/css/custom.php

@@ -2,6 +2,7 @@
 <?php
 require_once '../../includes/functions.php';
 $color = getColorOpt();
+$allCss = 'all.css';
 ?>
 /*
 Theme Name: RaspAP default
@@ -11,7 +12,7 @@ Description: Default theme for RaspAP
 License: GNU General Public License v3.0
 */
 
-@import url('all.css');
+@import url('<?= $allCss ?>?v=<?= filemtime($allCss); ?>');
 
 :root {
   --raspap-theme-color: <?php echo htmlspecialchars($color, ENT_QUOTES, 'UTF-8'); ?>;