Which PHP version do you use? Check your PHP-error logs for "syntax error, unexpected '/' in ../../lib/tpl/YOURTPL/style.ini"-Errors. If you find such erros, your PHP is running into
Bug 49692.
If so, possible solutions are:
* Best option: upgrade PHP.
* Alternative: use a template which does not use '/' in the CSS paths of the style.ini. There are workarounds if you are using the vector or monobook template: Cf.
vector README,
monobook README
Edit: update github-URL.