Allow a sub section of our variables to be set via the theme UI

[markconroy]

I think it might be good to allow editors to set some variables via the theme - for simple one-off changes that might last a day or two, e.g. accent colour (you might want to change it to red for christmas!), fonts (if we provide a set of prepared fonts like in microsites), etc.

This will also give us a good base to try bring live previews module to LGD core.