Typemill ships with a standard theme called "Cyanine". Cyanine is an univeral theme that you can use for documentation, as well as for any other kind of text-work. You can configure the theme in the author panel under settings and themes.

For more themes, head over to the Typemill Theme Directory.

Screenshot of the theme directory of Typemill

If there is a new version of the theme available, then you will see an update banner in the theme area of the author panel. To update a theme, open your FTP-software, go to the theme folder of your typemill installation, delete the old folder of your theme (e.g. /themes/typemill), and upload the new folder.

If you are a developer or web designer, you can easily create your own theme with the Twig templating language. Please read the theme documentation for more details.