I think Friendica, including the frontend, is way too complex currently so I'd l

I think Friendica, including the frontend, is way too complex currently so I'd like to reduce that complexity. Really I'd like to get down to a single "theme", and before your work on Vier I would have suggested removing Vier, so Frio is the only option. Now with Booklook my hope is thatat some point it could replace Vier, and possibly Frio, too.

Then themes would be pure CSS, like Bookface, instead of being different templates. Separate templates makes things a lot more complicated to maintain IMO.

For Frio I'm personally fine with Bootstrap and I'd like to "outsource" as much work as possible to it so we have less code to maintain ourselves, but I think Frio relies too much on JavaScript and really does some strange things with it, fx. making changes via JS that should be template changes. So ideally for me we'd greatly reduce the amount of JS in Frio, and also, as you said elsewhere, move away from JQuery to vanilla JavaScript.
Finally a lot of things that JavaScript used to be needed for, CSS can now handle, so I'd like to use CSS there instead.

I think you I saw you also mention this, but I think the reliance on different HTML for desktop/mobile is not great either. It would be better if we could use the same HTML and restyle it via CSS.

Currently I'm generally working on Frio and I have a lot of plans for it. Some of those are my own ideas or based on what other platforms do, and some are essentially just doing what Bookface already does.