lib/api: Make theme paths relative (#6142)

* Update theme.css

* Update syncthingController.js

gui/default/assets/css/theme.css

@@ -7,5 +7,5 @@
 
 */
 
-@import "/theme-assets/dark/assets/css/theme.css" screen and (prefers-color-scheme: dark);
-@import "/theme-assets/light/assets/css/theme.css" (prefers-color-scheme: light), (prefers-color-scheme: no-preference);
+@import "../../theme-assets/dark/assets/css/theme.css" screen and (prefers-color-scheme: dark);
+@import "../../theme-assets/light/assets/css/theme.css" (prefers-color-scheme: light), (prefers-color-scheme: no-preference);

gui/default/syncthing/core/syncthingController.js

@@ -663,7 +663,7 @@ angular.module('syncthing.core')
                     document.documentElement.style.display = 'none';
                     document.head.insertAdjacentHTML(
                       'beforeend',
-                      '<link id="fallback-theme-css" rel="stylesheet" href="/theme-assets/light/assets/css/theme.css" onload="document.documentElement.style.display = \'\'">'
+                      '<link id="fallback-theme-css" rel="stylesheet" href="theme-assets/light/assets/css/theme.css" onload="document.documentElement.style.display = \'\'">'
                     );
                 }
             }