Set header font-weights to 'normal' (400)

After 'Open Sans' was removed, I believe the font weights should be set back to normal/400 for headings instead of 300 which suited Open Sans. Some might argue it should be 'bold'/700 for headings as that's browsers' default.
Merge pull request #4192 from meredevelopment/master

Set header font-weights to 'normal' (400)