FireMonkey (while slow and bloated), seemed like an initial good idea, but seems poorly implemented. Should we look at building something similar?
(and browsers are not slow and bloated ? I think if it is very general, it will always be that. If try something yourself, keep it focussed and don't generalize to early)
It is a very, very difficult road. Moreover it is not all or nothing, you can use owner drawn components in an otherwise standard widget set. Windows has options to modify the standard bar that can be used.
Discussions about owner drawn vs standard widgetset have raged multiple times, I suggest you search the archives, but the main reasons are about nativeness of the application and the barrier to actually do and maintain the result.
Besides the already mentioned attempts (some of which are of the partial kind) you could also look to third party open source owner drawn libraries like wxwidgets and IUP.
There is also msegui and fpgui that tried to do something like it for FPC, and some lucky licensees dabbled with the commercial package that later became firemonkey after Embarcadero bought it. (or hired the author, I don't know exactly)
Afaik fpgui had some success in the POS world.
To make something simple is easy, but to get it to a decent level and to maintain it, that is harder.