Comment 4 for bug 826441

It's a bad idea to use user-stylesheet-uri in this manner because the user may have already set it in their preferences, or other code (e.g. extensions) may want to use it as well. I think if midori is going to programmatically assign user-stylesheet-uri to apply global theming, it should do so in such a way that each component's changes to the property are reversible and act as additions to the old value rather than simple assignments.

This could be achieved via a wrapper with add and remove functions for style rulesets and which composes a user-stylesheet-uri from the known active rulesets and the value of the setting from config, but any solution should ensure that midori does not destroy user-specified values for the setting--it's the most effective way to unconditionally apply styling, as userstyles are known to "flash" and require javascript.