Um die Splash Themes von sddm (und auch die Themes nach dem Login) unter qt6 zum Laufen zu bringen,
sind (wenigstens) die folgenden Modifikationen nötig:
$ sudo modprobe install qml6-module-qt5compat-graphicaleffects:amd64
Anschließend sind Zeilen der Form
import QtGraphicalEffects x.y
überall in Splash.qml bzw. Main.qml und anderen qml Files durch
import Qt5Compat.GraphicalEffects
zu ersetzen. Hier ist ein Beisiel für die notwendigen Maßnahmen
an sddm-theme-giants, welches mit
$ sudo apt-get install sddm-theme-giants
installiert werden kann (falls noch nicht vorhanden):
$ diff -r usr/share/sddm/themes/giants /usr/share/sddm/themes/giants
diff -r usr/share/sddm/themes/giants/components/PwBox.qml /usr/share/sddm/themes/giants/components/PwBox.qml
28,30c28,29
< import QtQuick.Controls.Styles 1.4
< import QtGraphicalEffects 1.0
< import QtQuick.Controls 1.4
---
> import Qt5Compat.GraphicalEffects
> import QtQuick.Controls 2.0
diff -r usr/share/sddm/themes/giants/components/ToolTip.qml /usr/share/sddm/themes/giants/components/ToolTip.qml
26,28c26,28
< import QtQuick.Controls 1.1
< import QtGraphicalEffects 1.0
<
---
> import QtQuick.Controls 2.0
> import Qt5Compat.GraphicalEffects
>
diff -r usr/share/sddm/themes/giants/Main.qml /usr/share/sddm/themes/giants/Main.qml
27,29c27,29
< import QtQuick 2.9
< import QtQuick.Controls 2.4
< import QtGraphicalEffects 1.0
---
> import QtQuick 2.15
> import QtQuick.Controls 2.15
> import Qt5Compat.GraphicalEffects
55c55
< onLoginFailed: {
---
> function onLoginFailed() {
diff -r usr/share/sddm/themes/giants/metadata.desktop /usr/share/sddm/themes/giants/metadata.desktop
15a16
> QtVersion=6
diff -r usr/share/sddm/themes/giants/theme.conf /usr/share/sddm/themes/giants/theme.conf
2c2
< background=images/background.svg
---
> background=/usr/share/sddm/themes/giants/images/background.svg
HTH