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
The javascript of the do-plugin tries to update the container with the
plugin__do_pagetasks class. However, since our container has a different
structure than the icon produced by the do-plugin, this attempt fails
and leaves the icon in a broken state.
This commit changes the class of the container and thus ceases to be
affected by the do-plugin's javascript.
SPR-946
