There aren't any themes for 3.90 M33 now, but I went ahead and flashed the Organ 3.80 M33 theme onto my 3.90 M33. What happened? It worked perfectly and no semi-brick.
Why did this work? I think It worked due to 3.90 M33 not being any different from 3.80 if I have a original PSP (phat). I only tried this on Phat, so I wouldn't recommend trying it on Slim, also if your gonna flash a theme from 3.80, make sure to backup FLASH0!