[DankActions] add theme support

While I'm looking forward to this (https://github.com/AvengeMedia/dms-plugins/issues/22) being implemented, I would prefer full theme support so that any custom action widgets would look the same as the other plugins.

Start by reading the referenced issue #22 and the DankMaterialShell plugin code that defines custom action widgets. Identify how existing plugins receive and apply themes, then determine the full theme-support scope needed for action widgets to match them. Done means custom action widgets render consistently with the other plugins across supported themes.

Written by the indexing model from the issue text.