How can I set a sitewide variable affecting all my .erb pages rather than inside a partial? Also, how do I set erubis as the default templating engine? The website doesn’t seem to document the available options.
There is no ‘default’ templating engine.
If you want a template to use erubis, suffix it with .erubis ( http://middlemanapp.com/basics/templates/ )
For a page, suffix with .html.erubis
While you technically can use different template languages for different templates, it is not recommended since this sometimes causes bugs. For the pages on the other hand, it seams that you can use whatever you want and mix freely.
Sitewide variable:
In config.rb:
set sitewide_variable, 'Use ”sitewide_variable” in a template to get this string'