Improve facet strips appearance in theme_bw

Make the border thinner, to match the thin border of the panel itself. The
border of the panel is thin because it is clipped so it is not possible to
match it exactly but this is already an improvement over the previous version.
Improve appearance of theme_classic

Facet strips were filled and legend keys were grey while all the rest is
black and white and as minimal as possible. Changes are made to match that
feel.
Add NEWS bit
Add PR number of NEWS bit for theme improvements
Merge pull request #934 from jiho/theme_improvements

Small (but important) improvement to some themes