Theme update: responsive date tabs

Pushed another small update to the theme this afternoon. Note the snazzy new date tabs to the left (shamelessly "inspired" by very similar looking things on Remy Sharps blog).

When the view gets below 480px width they switch into normal in-line dates so there's no wasted space all down the side allowing maximum content width on devices.

I've also done a few other updates like the font and various minor tidy-ups, baby steps but getting there :)