Read theme.conf of themes
Summary:
Otherwise kcm_sddm ignores values in there. Previously, a theme with a default
background image reverts to a color background on save.
Test Plan: Ran kcm_sddm and saved without changes. The background type is still image.
Reviewers: Plasma
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D12323