Yeah... why fiddle with GTK_Window_Context_Handler_Ptr if you can fiddle with the intricacies of CSS layouts instead?
A lot of the color fiddling etc. that web apps due is due to
1.) Graphic designers having no other way to go, now that print is dead
2.) Corporate Identity (which is of lower priority than platform identity)
3.) Selling stuff (not needed as much with desktop apps)
And since most folks end up supporting the web as a first class platform you have to BYOD there anyway. Nobody is out there making a JS framework to emulate WinForms.
Not everything has to or can be be cross-platform, not everything has to be web-visible. And I'd argue that serving the same view to everyone is quite often a bad idea. (Heck, in the web-case, it's not even just the view)