Supporting themes from all apps and unit tests

Need to be able to easily handle theming all apps, not just the media wall. Now the themer takes a CSS string as an option in the constructor. When `applyTheme` is called, it takes the theme object and replaces the `var(--variable)` string.
Worth noting, I guess, that this won't be used if you also provide `opts.prefix`. Maybe only support opts.prefix and push any knowledge about packageAttribute stuff to modules one level up.