We've had a set of %placeholders in our base styles for quite a while
but not butten the bullet to use them. This begins to use them.
We had to make a small amount of tweaks to base whilst doing this, but
its as we'd prefer there to be as few font placeholders as possible. We
might/should be able to reduce these further at somepoint, or
potentially rename them. We currently have six header fonts (or 4 header
fonts/2 strong body fonts) and 3 body fonts.
We also noticed an empty CSS file and deleted that while we were here.
We also noticed that the bottom border of structure tabs was a pixel
larger than ours so we tweaked that here also.
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