if you are talking skins like in most FPS, then no. all graphics reside server side so you have no means to change those.
if you are refering to a custom UI, then sure go for it. i used to use our guild UI as it had everything i needed/wanted in my UI.
% for target with enlarged target bar
% for spell casting timmer
full discription of spell gems (gem pic then next to it the name of the spell) for gem box, and buff/debuff box.
% xp
custom hotkey box that also had inventory slots.
xp bar without getting into your inventory
coin slots without getting into your inventory.
the only reason for opening your inventory was to move gear around that you had on.
for me that was a great UI. others have their own taste. you can customize it any way you want, but keep in mind every time SoE has a patch that involves a new xml file if you do not update your custome UI, you could be fubar until you fix it.