WowAce

No matter what I do, I can't change the background texture in general settings.

I've tried reloading the ui, relogging and create other profile but nothing works. It stays with the ugly gradient texture.

If you are speaking about the health-bar texture, you are changing the wrong option, go to:

Indicators -> health-bar -> Layout-Tab -> Change texture here

The general background texture, is the BACKGROUND TEXTURE the texture behind the health-bar (texture displayed when a player is almost dead).