Add theme class to the html

When a site theme is different than the default, it would be nice to add a class to the html or body tag, something like: theme_site_mytheme

Course themes may also apply, but those should be applied different because will only apply that course. theme_course_4_mytheme

Category themes can also occur but we can improve course WS to answer the category theme if it's not done.

This way we can apply different styles to themes, mostly icons that can improve the User experience.