Background color for sidebar headers

Summary:
kcm modules have backgroundColor as main background color.
The new systemsettings sidebar design has viewBackgroundColor as main background color.
Through this, the kcm modules look a little misplaced to me.

With this patch, the sidebar's headers have backgroundColor as their background color, matching the background
color of kcm modules. Thus, kcm modules look a little better embedded.

This might conflict other's tastes or even conflict VDG guidelines I am not aware of. So let me know what you
think.

Reviewers: #plasma_workspaces, mart

Reviewed By: #plasma_workspaces, mart

Subscribers: broulik, ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D8277

sidebar/package/contents/ui/CategoriesPage.qml

@@ -27,7 +27,8 @@ Kirigami.ScrollablePage {
     id: mainColumn
     Component.onCompleted: searchField.forceActiveFocus()
 
-    header: Item {
+    header: Rectangle {
+        color: Kirigami.Theme.backgroundColor
         width: mainColumn.width
         height: Kirigami.Units.gridUnit * 2.5
         RowLayout {