Distros SHOULD be theming all applications. Once thing a distro can do well is create a theme for their distro so everything looks the same.

Maybe distros are not doing a good job of this, but that is one thing they have the power to do. QT, GTK2, SDL, wxwidgets, and more toolkits I can't even think of should all fit together on my desktop and look the same. xfce, gnome, and kde all have some nice apps, they should mix and match. This is a HARD problem, but that isn't an excuse to not face it.