Fix CSS styles for new/deleted files in preview diff screens

Daisydiff may insert span tags into the inlined CSS to highlight it
with green/red background. As result the CSS is unparseable and is
ignored. Since it's not possible to avoid this, fix it after the fact.

Change-Id: Ib4d7dc105888954ccf149ced0abb0078f032607c
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>