Banners: added the em function to banners

After the SCSS was reorganized, the em function has been left out of the scope of the banner styles. This changeset brings it back, thanks to @mikeyarce for submitting an original #1242 pull request and to @MichaelArestad for pointing out a less invasive solution.

This replaces the solution that was suggested in #1242.

Note: It looks like for some reason previous changes to styles were not translated to RTL, hence there are some modifications to RTL styles in this pull request that are not related to banners. I'm not sure if I could have left them out of that commit.