At a recent WordPress Wednesday Meetup, Don Brown had a great question that forced me to realize I was wrong in something I’d blithely assumed about the Customizer. Or, more accurately, never really stopped to think about. As you doubtless know (since you, my dear readers, generally know everything and I am perpetually just trying to catch up to you), in December of 2016, WordPress 4.7 arrived with the new option in the Customizer to let us add Additional CSS. (Appearance > Customize > Additional CSS option)
Well, it sounds silly to say it out loud, but I had assumed that the “Additional CSS” you added via the Customizer was stored in the database, not dependent on the theme it was enhancing. My friend Don gently questioned that assumption until we saw that, of course, all CSS added is attached to the particular theme you’re adding it to, and when you switch themes, that CSS doesn’t magically appear. Thanks for helping to clarify this in my mind, always good to learn something new! Looking forward to seeing you at the next Bucks County Marketing and WordPress Consortium meeting!