the background of the view selector list should be transparent

Summary:
avoid having a big rectangle with a default color not following the
color theme

BUG: 408435

Test Plan: now playlist is correct with Breeze and BreezeDark

Reviewers: Elisa, zanny, ngraham

Reviewed By: Elisa, zanny, ngraham

Subscribers: neousr, ngraham, zanny

Differential Revision: https://phabricator.kde.org/D22086