Bug: Accordion color customization

[azuryte5]

Summary

graupl-accordion-background also changed the background of the gap between accordion items.

It would be better to be able to set accordion item content background, header background and footer background for futher theming customizations

Steps to reproduce

1. Set accordion-background variable or use preset default
2. entire accordion area is set, including gaps between accordion items

Expected Behaviour

Should be able to set, or create accordions with different background areas.

[NickDJM]

Not a bug. This works as intended. However, we can add the other options as well