[sddm-theme/lock screen] Improve appearance in software rendering mode (outlines and icons)

Summary: This patch improves the appearance of the sddm theme and the lock screen with software rendering in place (QT_QUICK_BACKEND=software) by adding an outline to the action buttons, correcting the error that prevented the username outline from appearing and by adding a background to the action buttons that are otherwise impossible to see on top of a very bright background in software rendering mode.

Test Plan:
Before (white background):

After (white background):

Before (Next):

After (Next):

Reviewers: Plasma, VDG, ngraham, filipf

Reviewed By: VDG, ngraham

Subscribers: plasma-devel

Tags: Plasma

Maniphest Tasks: T10325

Differential Revision: https://phabricator.kde.org/D19609