how to add a new theme?

see title

If you mean adding a theme to your Giant Bomb profile, that's not possible (yet). If you mean something else... Um, clarify.

a theme for a game lika action, rpg ect...

On the PS3? On the 360? On GiantBomb?

Wtf...

"a theme for a game lika action, rpg ect...
"

no no you got me wrong
its for in the game description

You can't, there is a drop down box and you choose the theme for the game...

:S