Bug 32022: (follow-up) Ensure action background takes presidence

Bug 32022: (follow-up) Ensure action background takes presidence

The action class sets the background to transparent, but because it's
often used in conjunction with the fieldset which includes the card
mixin the background rule was getting overridden to white.

Test plan
1. Consider the same cases as the original patch
2. Confirm that the last test now shows the buttons outside of the card
   but nicely spaced to clearly belong to it.

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>