Theming actually appears to be pretty simple. It's based on CSS overrides, so you'll have to do a little of your own investigation to know what the tags and such are.

There are a couple of approaches I can see to this. The themes are saved in the public/assets/themes folder. You have two in there by default. Default and sandbox. The sandbox theme is actually an empty stylesheet that doesn't override anything in the main stylesheet. The Default has the CSS and image elements for the default style.

If you want to do a lot of work, start with the sandbox. You can either use something like CSS Edit to find the tags for the elements you want to adjust, or look in the default CSS file to see what it includes. You could also try duplicating the default theme and editing it to suit your needs, which might take less time but would give you less of a unique look.

I haven't tried it, but it appears you can just create a folder with a file called theme.css inside it in that themes folder, and that should make it show up in aC. Or you could just start editing the sandbox theme.css file, since there's nothing in it to start with.