I have had instances where I have had to modify the CSS of a WordPress Theme to make some things work the way I want.
To accomplish this I have been customizing the Theme style sheet. In the Theme I'm using now it is called style.css.
I need to upgrade WP to the newest version. I suspect that when I do that my custom CSS will go away.
I recall there is a technique to have your own custom style sheet that OVERRIDES the theme style settings & it not changes by upgrading WordPress or the Theme.
I need a reference on how to create that & use it in a specific site.