Apart from using the webview less there are a couple of opportunities to improve the look and feel of the webview instances we need.
Mostly by injection a css that sets background color/font/...
See http://stackoverflow.com/questions/41931172/webengineview-set-custom-font-to-html-content
and kdepim/messageviewer/viewer/csshelperbase.cpp, csshelper.cpp, viewer.cpp