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