Part of what gives the default KDE theme that feel you describe I think is its usage of transparent grays. Generally when you do transparency in UI it needs to

Part of what gives the default KDE theme that feel you describe I think is its usage of transparent grays. Generally when you do transparency in UI it needs to be transparent white or transparent black (though in some case, dark transparent gray can work), because transparent mid-grays make everything feel kinda dingy, with whatever's showing through feeling like it's being seen through a thick fog on a dark rainy day.