Made the split separator darker

This is a fix for #9146.

@RaymondLim I've made the separator a little louder for light and dark UI. Hopefully these will do otherwise it'll start breaking the visual hierarchy.
We should design for normal brightness so if it's not bright enough then you can turn the brightness up a notch. Normal brightness is pretty common for people that work on design primarily compared to other types of work where the screen color accuracy is not as critical.