Fix ridiculously small font size in overlays under MonoBook

Copy in the font-size:127%; rule from MonoBook's main.css which is applied to
div#globalWrapper

Caused by Id425c56d

Change-Id: I05502295b81c62fd1180dff860dea68d76c2dfa9
(cherry picked from commit 8a1adef97c125b70a9d60ca3e7f91dbf86d59f52)