Make header opacity configurable

[micw]

The background in page-header has a fixed "bg-black-40" class. It would be nice if that can be configured via variable

[ramiro]

cover_dimming_class in global Params already allows this for the home page. The proposed PR applies the same value to pages.

[micw]

Should it use a new parameter or just add the behavior using the existing param?

Edit: nevermind, I missed that the PR is already there and not to be written :-)