No, the first thing to do would be to open the Browser Developer tools (F12) Inspect the element, and check why your rule is not working.
If you see your rule but it is being overridden by another one, then trying !important is one thing you can try. Another would be to check if the rule is being overridden because the selector you chose where not optimal compared to the one used by the overriding rule.
If you do not see your rule, then the problem could be that you placed it in the wrong place or that you had a Syntax error in your rule, or a syntax errors in some other custom rules before that.