A question

Imagine, if you will, that I haven't worked in IT for 10 years and that I know nothing about CSSs (is there a plural?). I know it's a tough ask but let's pretend I'm really totally clueless about any sort of coding. What I need is an easy guide to CSSs or some kind soul to point me in the direction of one that's mostly like the format I have now but lets me change my banner. I'd like to learn to make them but I'm not sure where to begin, other than experimenting with one and making a hash of my journal while I experiment. Not sure I want to do that. So, can someone point me to a CSS for dummies?