Concerning basic settings vs. granularity…
Vray (for 3ds max at least) have a very nifty concept called UI modes:
Were the user can flip through 3 levels of ‘setting granularity’ - default, advanced & expert.
Down the road (when Blender 2.8 UI gets more refined) I personally think it could be interesting to implement something like this selected places in Blenders UI.
Maybe just a icon that flips between basic and advanced ‘setting granularity’.
Theme settings could be such a place.
As basic theme settings, I agree with you that the Unreal solution seems like a well-thought-out implementation (though I have not used the program).