Provide theme class for CSS (#7559)
* Provide theme class for CSS
* Replace more common characters with -
* Fix test error
* Make requested changes
* Make more requested changes
* Use htmlspecialchars_decode instead
* Replace whitespace
* One more whitespace character
* Simplify and move class to <html>
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Co-authored-by: Alexandre Alapetite <alexandre@alapetite.fr>