material: fix issue with cookie check for light/dark theme

Fixes #4812

Signed-off-by: Kevin Decherf <kevin@kdecherf.com>

app/Resources/static/themes/material/index.js

@@ -58,7 +58,7 @@ function darkTheme() {
     },
 
     exists() {
-      return document.cookie.split(';').map((cookie) => cookie.split('=')).filter((cookie) => cookie[0] === 'theme').length;
+      return document.cookie.split(';').some((cookie) => cookie.trim().startsWith(`${this.name}=`));
     },
   };
   const preferedColorScheme = {