The attached patch adds full theme support in the following way:

- move basic layout CSS into maavis_layout.css
- load maavis_layout.css as the default stylesheet
- move all colour CSS into maavis_colour.css and maavis_bw.css (for colour and black and white themes)
- remove static loading of both colour and bw css files and replace with dynamic loading of maavis_THEME.css where THEME is replaced with the value of the theme configuration property

Default behaviour remains unchanged. That is the old BW and Colour themes are unchanged. However, we are now able to add arbitrary themes as maavis_THEME.css files supplied as part of MAAVIS.

To demonstrate this I have added a maavis_dementia.css which uses black on yellow as the main colour theme.

Subsequent work (I'll open tickets) should enable loading of remote CSS files and for CSS files to be delivered in the users media folder.