I've never had any trouble changing fonts and text colors on any distro (in CSS/Code or View). But then I compile Sigil myself rather than use the packages from the repos. The repo versions often integrate Sigil with their own qt theming plugins. Perhaps they've broken something there. We don't do any specific platform theme integration in Sigil's base source ourselves. We leave that up to the distros. Too many distros and too many themes/styles to worry about.
The automatic Qt stylesheet support wasn't added to Sigil until v0.9.6. That's why your .qss file is being ignored by the 16.04 supplied 0.9.5. You should, however, still be able to force the use of particular Qt stylesheet by launching Qt applications with an additional command-line-parameter.
Code:
qt_application -stylesheet style.qss