The name of the .css file shouldn’t matter. It just needs to be a valid .css file and enabled in Settings. Of course, the CSS in the file also needs to be correct.
As for learning CSS, in another life I made websites, so had some basic knowledge. There are loads of guides. You could have a look here: CSS Tutorial, but in addition to “standard” CSS, Obsidian has its own flavor to take care of the different viewing modes. This is a good read:
