@stap that is so so helpful - thank you so much. I think I'm going to use 'Clean' as my base and use the info you've given me to tweak the colours to roughly match the Switch Dark Theme (I reckon I can get it quite similar in feel) - graphic design is my job.
Based on what I've read I think I'm better staying with stock firmware and this theme mod. I'd like to get some Gomba Colour action for a few GBC must plays like MGS, Zeldas and Wario games - but I've read scary things about quitting that emulator (instead of powering off after a few seconds) corrupting SD Cards (and therefore your whole GBA content).
So initially I fancied trying to play some GB/GBC, NGP and GG (even TG16 - but I can imagine how it handles the squishing of the graphics). But now I thinking to just enjoy it as a pure GBA playing Micro.
What do you think - have you bothered with other systems much?