A profile background is not a chat code. Pick a theme here, then open your xat profile settings and choose the matching background swatch. The preview shows exactly how that gradient looks behind your name and status.

All 15 official themes are shown: the free gradients (pg1-pg6) and the power themes (pg7-pg15). A power theme only shows to others if your account owns its power (Morgana, Kitsune, Halloween, Cogu, Lems, Stella or the Yellow ace).