There are two at the moment, you can switch them either in your profile or at the bottom of the page if you follow the Change Theme: NeoGAF 2018 - Dark link. There are plans to have more moving forwards but they just haven't been the priority in terms of where work has to be done yet.
I use the dark theme for this account and the light theme for my standard account, it makes it easier to remember who I am when doing that.
Used to use the light theme but I use dark theme now. Its way easier on the eyes when I wake up at random times at night and just look at Gaf on me phone. One night around 3am I had it on light theme before with my phone on the brightest settings once, and I swear I was blinded for a minute. Felt like a vampire walking into sunlight.