Explicitly set font-weight to 500 in CSS

Explicitly set font-weight to 500 in CSS

Bedstead's standard weight is 500, reflecting the fact that it's
naturally quite heavy.  CSS defaults to 400, so we were depending on
CSS's font-matching rules to fall back from 400 to 500.  That's fine
at the moment, where there is no 400-weight version.  But Bedstead
could be made lighter, and indeed the code to do it already exists.
And I hope that somewhere in the future is a variable Bedstead that
might allow for lighter weights.  So for future-proofing I think the
CSS should specify the weight that it actually wants.  Obviously I can
update my CSS if I ever introduce a 400 weight, but other people might
reasonably use my CSS as an example and I shouldn't lead them into
trouble.