Having played with both GTK and QT I've found most desktop GUI toolkits unpleasant. I can see why a lot of devs end up using tools like electron, while the javascript stack has it's problems on the whole I'd say that it's easier to get started building a UI using that tooling then trying to build a new gnome app.
So if there was a new GUI toolkit, something that could compete with electron and GTK, what could it look like?
I'd use capnproto to expose a simplified "DoM" set of interfaces for manipulating a scene-graph of objects. This would be vastly simplified over what the web provides, and the main way of laying objects out would be flexbox-derived.
I'd name all the colors in my theme, probably following bootstraps example of (primary secondary success danger warning info). Semantic names that tell you what the color should be used for, instead of saying (red green blue whatever).