Use header colors for custom toolbar on Main page

This will fall back to the current Window colors when using a color
scheme without Header colors, so there is no visual change right now.
However this ensures that we are prepared for the new Breeze color
scheme once it lands.

src/data/contents/ui/MainPage.qml

@@ -182,6 +182,8 @@ Kirigami.ScrollablePage {
     }
     header: QQC2.ToolBar {
         id: toolbar
+        Kirigami.Theme.colorSet: Kirigami.Theme.Header
+        Kirigami.Theme.inherit: false
         RowLayout {
             anchors.fill: parent
             Kirigami.SearchField {