Hello,
It sounds like you edited the plugin’s CSS files directly. It’s not a good practice to do that, for the reason you just described.
There are two ways to add CSS rules and have them stay there. Which one you choose is up to you:
1. Create a child theme for your theme. If you google “child theme”, there are multiple guides on how to do this. It’s super easy. Once you have a child theme, you can add all sorts of things on it that are specific to your site, including CSS rules and any random PHP functions that are only useful on your site.
2. Use the Customizer. Go to Appearance -> Customize -> Additional CSS. Add your CSS rules there and hit “Publish”.
When you add rules, make sure to observe rule specificity. You want your own rules to override the rules of the plugin, so your rules need to be more specific. Again, there’s multiple guides on the web on the topic. The one I usually recommend is this one: https://css-tricks.com/specifics-on-css-specificity/
Hope this helps. Again, please open a new thread if you have other questions.
with regards