Hello,
This problem is not really about the plugin, but here’s a few things that might be giving you problems with your CSS:
1. First of all, you should not add your rules to any existing file belonging to the plugin or theme because these files get overwritten on each update. This is why the recommended way is to use a child theme. Because the child theme is a separate component, it can remain as is even if other components are updated.
2. In the example you showed me above, there should be at least one space between the first closing curly bracket and the second selector.
3. There might be other rules on your page with a higher specificity that override your rules. To learn more about CSS specificity you can have a look at this fun tutorial.
In general, you can use your browser’s inspector feature to see which rules are actually applied to your elements. This can help you quickly debug your CSS.
You can also use the browser’s debug console to verify that your CSS file was actually loaded.
Best of luck!