It uses the colours you pick in your terminal. I made the website dark because I like that colour scheme, and it's what I look at all day, so copying it for the website was a natural fit :)
A few of the filetypes use colours in the 256-colour range, but these should work well too - the grey colour you see for non-existent '-' table cells in the columns view is the exact middle grey value (colour #244) because it's the only one that's guaranteed to work with light-backgrounded terminals.
Well, it's originally the Tomorrow Night theme, but I've tweaked all the colours to my own liking (such as making the bold colours slightly lighter) that it's not really officially that anymore. But it's a great theme, and the nearest.
A few of the filetypes use colours in the 256-colour range, but these should work well too - the grey colour you see for non-existent '-' table cells in the columns view is the exact middle grey value (colour #244) because it's the only one that's guaranteed to work with light-backgrounded terminals.