Hoi Pascal,
in general it works, but in some older templates there was mistake and custom.css is loaded before template styles :/ and that's why you don't see your custom CSS changes.
To fix it right, you have to edit index.php file. Are you OK with that?
OR
2nd idea: Inside Template Options > Custom Code > Before Head just paste that line
<link href="/new_gb/templates/politicas/css/custom.css" rel="stylesheet"> Save & Close.
then that .css file will be loaded after all template styles and javascript. Not perfect, but 100% helps.