You can set custom themes for the site yourself. First, go to your profile --> edit icon --> Edit Theme. There you can set text and panel colors, background image, etc. This will set up the theme on your personal profile.
Then, go to your profile settings --> "General" tab --> Site theme --> choose "my profile theme" from the drop down list. This will make the entire site have the same theme as your profile, when you browse it.
@Oreo321 This is an annoying thing to do because, well, you'd need to get the exact settings and picture.
BUT!
I found a work around and I managed to set my site theme to SA2B. I recorded a video explaining it. Sorry about the terrible microphone, my headphones broke a few days ago so I'm using my laptop mic.
Nice, I didn't knew it worked that way. That means, some of the custom site theme options are actually taken from themes of exist games on the site, and you can copy their settings. (Some of the options actually copy the themes of specific users, like "Milk").
I guess the best thing in this case would be, to have the option to set a custom site theme from any game of the site.
Turns out it doesn't stay though, it put it to Default. It stays for a bit but goes away after. Shame. Pretty stupid, too.
Edit: it does stay. I have been using the SA2B theme for months and I didn't even realize I had it on.