Edit src/vars.scss file. In particular, you'll want to consider the following variables:
$brand-primary
$text-font-family
$border-radius-base
Sass it up:
> sass src/index.scss > path/you/want/to/save/it/to/file.css
Upload to Zulip and have fun.
If you want to contribute, that's cool. Hit me with a pull request.
Otherwise, I may tinker with this some more as time goes on.