There have been some z-indexes that were removed and then reintroduced
through strange merges. This is an attempt to remove all z-indexes that
are unnecessary and to clearly state the purpose of the remaining
z-indexes in a comment to each index.
SPR-981
In the dokuwiki standard template __background__ is the variable
defining the background-color of the content. It is one of the
guaranteed placeholder that every template must provide. Hence we should
use it with the way it is used in the default template, so other plugins
can rely on it and look consistently.
Also some other base-dokuwiki styles still use it accordingly, for
example the footnotes.
SPR-954
