[Evolution] Theme fixes for white-on-white in HTML

[aguador <waterbearer54 gmx com>]

Milan, thank you for the fix for the webkitGTK+ changes in Evo 3.34.1.

As expected the fix works for the Adwaita theme, as well as the Vertex
theme. However, some themes may still not display them properly. For
those who like darker themes, the Shades-of-Gray themes has been
updated to work properly. Anyone using it or interested should download
the latest version from GitHub:
https://github.com/WernerFP/Shades-of-gray-theme. You will see the
change that was added by looking at closed issues.