Fixed dark theme not applying on the first time (#273)

Reviewed-on: #273
Reviewed-by: Mikolaj Widla <widlam@noreply.example.com>
Co-authored-by: Adam Bem <adam.bem@zoho.eu>
Co-committed-by: Adam Bem <adam.bem@zoho.eu>

Frontend/src/components/sidebar/SidebarComponent.vue

@@ -18,7 +18,10 @@ function changeLogoForTheme(){
 }
 
 function isDarkModeSet(){
-    return localStorage.theme == "dark";
+    if (localStorage.theme) 
+        return localStorage.theme == "dark";
+    else
+        return window.matchMedia && window.matchMedia('(prefers-color-scheme: dark)').matches;
 }
 
 function changeTheme(theme:string){