Great food, preferable dimly lit - girl's night birthday dinner in Santa Monica - Italian or other steak/fish options.
Eeek. Okay: Dim, Italian (but could be steak/fish), not too expensive but not too casual, great food....
Also in Brentwood is Villeta but you may struggle at the last minute with no rezzie and it can get spendy.
La Botte in Santa Monica is nice--I think the food up the street at Hosteria del Piccolo is better but that isn't dimly lit at all--and you could probably get in. Or you could try the original Piccolo.
It's not steak/fish or Italian but you might be able to squeeze into JiRaffe.
I can't keep myself from mentioning Capo, but I think it is going to be too pricey and they may not be able to accomodate you at the last minute (the place is small and fills up).