Theming: Fix styling of TargetSettingsWidget

If using the flat style also the TargetSettingsWidget should be flat.

Change-Id: I16147c9868e3e949a4d9aa0fedf85358504bde9e
Reviewed-by: Thorben Kroeger <thorbenkroeger@gmail.com>
Reviewed-by: Orgad Shaneh <orgads@gmail.com>