I'm going to merge this, but just FYI, you should be able to put UI test only CSS in https://github.com/piwik/piwik/blob/master/tests/resources/screenshot-override/override.css as well (to keep it apart from the CSS that is served).
I don't really like putting it there. That's why I added that class
uiTest for a while. Those kinda things should be defined in the plugin that defines the HTML etc. So when re remove the plugin, all code is gone and when we need to work on a plugin, all code is in one place as well and not spread across multiple places.
Also plugin developers cannot use this
override.css so we should only do it, how plugin developers have to do it as well to have good examples and re "eat your own dog food" etc.