Squashed 'beamerthemes/' changes from 0346ea1..65b44be ...

65b44be • Set definition of “page number in head/foot” colour back to what it should be, as apparently whatever bug that made it not work before has now been fixed.

git-subtree-dir: beamerthemes
git-subtree-split: 65b44be

Squashed 'beamerthemes/' changes from 2d36aa0..177a717 ...

177a717 • Added Otago “plain” theme.

git-subtree-dir: beamerthemes
git-subtree-split: 177a717