I like the night theme so I use it. I can't remember how I feel about the other themes currently. I really like dark colors in general so I usually use the night theme unless I'm used to other defaults (like on DA). Also, the white/default one hurts my eyes a whole lot so I hate logging out to test stuff.
i use the pink one! simply because i think its the prettiest and goes best with the coding for one of my characters~
Pink Velvet, because I try to go with a red theme on every site I'm able to customize, and this was the closest to red that was available.
I just use the original one because the others all feel super different from it and would probably be kind of jarring for me to try and get used to. they're super pretty but I'd rather stick to what I'm already familiar and comfortable with. plus, if my profiles end up looking ugly on them, I'd rather not have to redo them (which I probably would for the sake of having them look good)
Default cause I like light themes for most sites and apps I use (except for discord......... light mode on discord is so jarring especially with the random dark elements LOL) and I also like the color blue, simple as that haha