Support standard skin-invert class as well as mw-invert

The math equations in the article
https://en.m.wikipedia.org/wiki/0.999...?minervanightmode=1
are not showing when rendered via MathML.

After a quick look I noticed this is using mw-invert rather than
skin-invert class.

Change-Id: I7d1722c49227705fbfc0cf093c7ebe49eaeebcc5
1 file changed