Pulls the monospace font rule in gr-diff into :host. The monospace font
should apply to everywhere by default, e.g. also to the context control
rows.
Before https://i.imgur.com/oMSfhC5.png
After: https://i.imgur.com/2PqhxKf.png
Also fix the black color of the hyphens in the context control rows.
Change-Id: I8ee16b46da6dd0454d0b8991cc0836d7a4550f8d