Almost every program does custom drawing, whether it’s a little graph, or a sound sensor or whatever. A web browser like using here the entire HTML DOM is self-rendered, right down to my little text editor Kate does custom rendering in the minimap and some plugins. This is not a rare thing.
Except things are done in different ways because they have different feature sets, you can’t map everything from one style to another, there are a lot of incompatibilities in the designs.
Valve has on KDE, lol.
Not last I checked, Safari is the IE of the modern times, always behind and broken and slow.