The modal window? If you're just wanting to use what we have and tweak it a bit, in the developer settings for your game (on Clay.io) we allow custom CSS that can override ours to tweak the existing styles.
If you're planning on building your own modal window, separate from Clay.io, there are several open-source options out there, but you can also use something like Chrome's Web Inspector to see how the CSS for ours is setup :)