If you still want to learn it, these two articles are good:

• Complete basics for how to use userChrome.css: https://www.userchrome.org/how-create-userchrome-css.html
• How to add own rules: https://www.userchrome.org/adding-style-recipes-userchrome-css.html

The video in the second article is important. Near the end of the video, they show the Browser Toolbox. With that, you can select a UI element in Firefox’s UI to find out what the element’s ID is. You will need the ID for writing the rule.