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
  index.html, although it is likely not a necessary condition. 
* Themes must be linked in index.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 NURDS, 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.
* Do not use CSS variables for other reasons: when dumping the current
  scene to an SVG, CSS variables will be ignored by current browsers.
* If you use pictures, either include a full URL or embed the whole
  picture in the CSS. Otherwise it will not be exported when dumping
  to SVG.