Beginner level help requested: adding text highlight colour to site title, tag pages?

Hi folks, brand new here and a bit over my head but ready to learn stuff. I’m working on our new ghost site (ghost pro with galerie theme) and I’d really like to be able to put a black highlight colour behind the white text for both: our homepage site title and description, and for tag pages on the tag name and tag description.

The reason I want to be able to do this is the text is currently pretty illegible on top of the images we want to use.

I know this must seem so simple to folks here so thanks in advance for your patience, and happy ny!