The code you were trying initially should work. Perhaps there is an issue with the CSS code that you have in the Global CSS such as some CSS syntax error that is stopping the CSS code you are adding to work as expected.
To verify that, please copy all the codes that are in the Global CSS and paste it here.
It should provide some information if there are any CSS error that is causing the issue that you have to fix so that the CSS codes will work.
If there is not issue with the CSS but the code is still not working, please provide us with the admin access to your site in a Secure Note: