Thank you all for your comments and perspectives.
shadav, I am with you (and SychO, previously) on the idea of using the inspect tool (Brave Browser/Linux). I was aware I could get the current colors in there, but not that I could alter them as a preview...thank you for that tip.
I actually had used inspect to successfully make several color mods using the SMF Admin/modify themes area, and the preview tool was making it even easier to ensure I was getting things right.
But I hadn't copied the modified index.css contents into a local file before attempting to save it...so the original file remained unaltered with that error, no impact on production (good or bad, lol)...but I had no backup of the mods.
SychO, I appreciate your way of gently guiding people to good solutions. Since my last post I found that I also have a edit feature as part of my host's File Manager. I tested it (added a comment, saved, viewed...then removed when all of that was successful). As an added bonus it provides line numbers, which I find helpful.
But as all three of you both pointed out, I agree it's wisest to develop off of production.
Kindred, you're kindness is showing through again. In a perfect world I'd have a second environment (Dev) where I could create and test my design before pushing it to production...and perhaps I will soon. In the meantime, I'm creating lots of backups of production, so if I mess it up I can at least get back to where I started.
I appreciate that all three of you are trying to help me progress, but pointing out the potential pitfalls too. This is a really good place to learn (and maybe someday when I'm ready, I can share some too).