Added dark mode files for studio

Added dark mode CSS files for all studio pages.

Now, this one's a bit janky. The studio CSS files 'bootstrap.min' and 'style-dashboard' already had settings for dark mode in them. 

The problem is that the PHP files for the studio sites:
app.blade.php, footer.blade.php, guest.blade.php, sidebar.blade.php and a few others specify the light classes in the PHP directly. 

As I didn't want to create separate PHP pages for dark and light mode, I simply replaced the light mode values in the CSS files with the dark mode ones. This means in the files the classes are still labeled as light mode but are in fact for the dark mode.

Selection for the dark mode will be handled the same as on the other pages, with JavaScript and a cookie (see next commit).

Author: JulianPrieber