Things you need to know before designing your own theme:
* Every theme is a CSS file whose name starts with 'theme-'.
* These files are typically stored in the same directory as
  urdumb.html, although it is likely not a necessary condition. 
* Themes must be linked in urdumb.html using something like
   <link rel="stylesheet" href="theme-insertthemenamehere.css" />
* Take inspiration from existing themes: copy the file, change the
  values.
* When a theme is selected in URDUMB, the team colours will be set using
  CSS variables --preferred-team-A-colour and
  --preferred-team-B-colour. The user can still change colours after
  loading the theme.