@bloodchen the core team is mainly focused on developing one standard basic theme that hopefully is easy to modify yourself. We've been pumping out tons of custom themes for our enterprise clients, at least half a dozen per year on average (I mean, we also have to eat.). We have a small team that works on theming (see what I did there), so naturally that is the state of freely available open source themes at the moment.
Anyway, the next theme will be based on react and/or vue (your choice) and I hope that we will be having more open source themers in the future.
NodeBB's cache buster is based on its current git hash (so if you've upgraded it will try to send you the new CSS and JS, which works well, but it remains the same if switching themes, which results in your issue)
I suppose the dev team hasn't run into this because we usually run with cache disabled while testing (and because Persona is so awesome so why switch? Kidding)