Moved change theme out of onload function

Moved this out of the window.onload function as the theme change needs
to run during load rather than 'on' in order to prevent page flickering
on reload
First pass at design change

Settings toggle and links are now in a bar at the bottom of the screen.
Settings card now appears as an overlay. Still needs some work and the
markup is a WIP. Light theme currently looks a bit rubbish too, but the
other themes are ok