A first general comment is: well done, from a code perspective, there is very little to say (only few inline comments).

I tried to build a ZIM with the UI inside and it is working (only discovered #226 and #227 but these are "details")

From a higher level perspective, I wonder if we should fine-tune a little bit the UI, it is quite visible that you've deviated from original design. I'm however not sure it is the right moment to do it, mostly because I'm not sure that your current design is worse than original one. I suggest that we keep the design as-is for now, and we then organize a session with designers once the whole UI is ready and with a real channel to discuss the details that need to be fixed and the ones we do not care about. We should open an issue about this, something like "Fine-tune UI visual details"

LGTM, thank you!