Linux and theming

cant really help you with howtos or anything cause i don't use them but im sure you can find a plethora of info at these sites

http://www.gnome-look.org/
http://www.kde-look.org/
http://art.gnome.org/

Well, the "windowthemes" are themes for the windowmanagers, standard windowmanagers coming with them...
KDE: kwin
Gnome: metacity

You can still use any other windowmanager you want with both DEs.

For the apps you have to theme the toolkits
KDE: qt
Gnome: GTK
which do not belong to the DEs. .oO( Everytime someone sorts GTK as a subcategory of Gnome I want to scream. :P)

I have no experience with kwin and qt, because I never have and never will use KDE.

For gtk and metacity you can find tutorials here: http://live.gnome.org/GnomeArt/Tutorials

Personally, I prefer XFCE as my DE. It uses its own window manager and GTK.

http://www.xfce.org/

enlightenment has a reputation for being pretty.

i use e-17, it has all kinds of cool effects.

http://www.get-e.org