If you are adding code to your theme there most likely is something loading after it that is overriding your change.

You can check your theme .css and make sure you remove the styles you don’t want or that your new .css comes after it in the document.

Sample:
H2{color:green;}
H2{color:blue;}

Blue will win because it’s loading last.

If you .css has to do with a plugin, sometimes they load after the style sheet too.

Does this help?