Thank you so much @merefield for clarifying that!
In this instance, I’m just wanting only to make super-minor CSS tweaks, not to establish or distribute a new theme.
For some unknown reason, the whole Git thing just scrambled-eggs my brain with a very unfortunate barrier (which I’m working hard to overcome!)
There might be others here in this community like me who (currently in their learning process) can only edit a certain amount of CSS which is only accessible through the Custom CSS/HTML settings in the default theme.