Fix breeze dialog background with Qt 5.12.2

Summary:
style elements without type="text/css" were ignored before, but now they act as
if type was set. Set id and style properly to restore the working behaviour.

BUG: 405548

Test Plan:
Installed new files on a system with Qt 5.12.2, wiped the SVG cache
and it looks fine again.

Reviewers: Plasma, rooty

Reviewed By: rooty

Subscribers: acooligan, filipf, heikobecker, falqueto, rooty, Zren, arojas, rikmills, asturmlechner, kde-frameworks-devel, plasma-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D19821