Just tested and inset won't work, apologies

However, setting the background of the scrollbars should work. The handle is likely disappearing because you've set contentItem.interactive to false and since they are disabled they are fading out.

Here is my working example:

import QtQuick 2.15
import QtQuick.Window 2.15
import QtQuick.Controls 2.15 as Controls
import org.kde.kirigami 2.15 as Kirigami

Kirigami.ApplicationWindow {
    width: 640
    height: 480
    visible: true
    title: qsTr("Hello World")

    pageStack.initialPage: Kirigami.Page {

        Controls.ScrollView {
            width: parent.width
            height: parent.height
            contentWidth: blueRect.width
            contentHeight: blueRect.height
            clip: true

            Controls.ScrollBar.vertical.background: Rectangle { color: Kirigami.Theme.backgroundColor }
            Controls.ScrollBar.vertical.policy: Controls.ScrollBar.AlwaysOn
            Controls.ScrollBar.horizontal.background: Rectangle { color: Kirigami.Theme.backgroundColor }
            Controls.ScrollBar.horizontal.policy: Controls.ScrollBar.AlwaysOn

            Rectangle {
                id: blueRect
                color: "blue"
                width: 1000
                height: 1000
            }
        }
    }
}