Themes
To re-use layouts and styling across multiple projects, you can create a theme.
A theme is really nothing more than a separate directory of content which gets combined (pun intended) with the content for your current site.
To use a theme,
create theme directory next to your content directory.
When you build the site, Combine will automatically merge the theme/content and content directories. To overwrite files from the theme, simply use the same path (i.e. content/_sidebar.html will overwrite theme/content/_sidebar.html).
Sharing themes
Because theme is just a directory,
a useful way to share it across repos is to use something like git submodules.
Using other directories
Using a directory named theme is a pattern that Combine detects automatically,
but you can change the name or even combine multiple directories by setting the content_paths.