Be careful about your choice of default GTK and Qt themes!

Currently most distros use Adwaita for GTK2/3 with Gnome, and Oxygen for all toolkits with KDE. These are really heavy themes. The GTK version of Oxygen in particular is an awful CPU hog on weak hardware, to the point of causing responsiveness issues even under e.g. Ratpoison.

My advice is therefore:

- For Gnome/Xfce/Mate -> Use Clearlooks-Phenix as a default theme. It looks nice enough, and makes Firefox usable again on my netbook, which is something.

- For KDE -> This is harder because there aren't many consistent themes. At bare minimum, the Raleigh GTK2 theme can inherit Qt colors, which makes it look almost passable with e.g. the Win9x or Plastique engines; unfortunately the default GTK3 theme cannot inherit colors last I checked. Best would probably be a combination of the Cleanlooks engine and Clearlooks-Phenix. Alternatively Qt can be made to emulate GTK widgets. Surprisingly this usually performs better than the Oxygen engine, though it can look weird in places.

tl;dr: Eyecandy always has a steep price tag. If you're creating your own Linux spin or desktop distro, be cautious about using eyecandy-saturated themes as the defaults, especially if you're aiming for low-end computers.