Set up a writable overlay for SDDM themes

Allows changing the SDDM wallpaper and the installation of new themes. This is temporary until plasma-login-manager is adopted.

mkosi.extra/usr/lib/systemd/system-preset/50-kde-linux.preset

@@ -21,6 +21,7 @@ enable tuned-ppd.service
 enable thermald.service
 enable pcscd.socket
 enable kde-linux-configure-firefox.path
+enable kde-linux-sddm-themes.mount
 
 # nvidia specific services (shouldn't affect non-nvidia hardware)
 enable nvidia-suspend.service

mkosi.extra/usr/lib/systemd/system/kde-linux-sddm-themes.mount

@@ -0,0 +1,22 @@
+# SPDX-License-Identifier: GPL-2.0-only OR GPL-3.0-only OR LicenseRef-KDE-Accepted-GPL
+# SPDX-FileCopyrightText: 2025 Thomas Duckworth <tduck@filotimoproject.org>
+
+# Allows changing the SDDM wallpaper and the installation of new themes.
+# Temporary until plasma-login-manager is adopted.
+# Also see https://github.com/sddm/sddm/issues/1561
+
+[Unit]
+Description=Set up a writable SDDM themes overlay
+RequiresMountsFor=/usr /var
+ConditionPathExists=/usr/share/sddm
+ConditionPathExists=/var/sddm_themes/themes
+ConditionPathExists=/var/sddm_themes/themes.work
+
+[Mount]
+Type=overlay
+What=overlay
+Where=/usr/share/sddm/themes
+Options=lowerdir=/usr/share/sddm/themes,upperdir=/var/sddm_themes/themes,workdir=/var/sddm_themes/themes.work
+
+[Install]
+WantedBy=multi-user.target

mkosi.extra/usr/lib/tmpfiles.d/sddmthemes.conf

@@ -0,0 +1,9 @@
+# SPDX-License-Identifier: GPL-2.0-only OR GPL-3.0-only OR LicenseRef-KDE-Accepted-GPL
+# SPDX-FileCopyrightText: none
+
+# Allows changing the SDDM wallpaper and the installation of new themes.
+# Temporary until plasma-login-manager is adopted.
+# Also see https://github.com/sddm/sddm/issues/1561
+
+d /var/sddm_themes/themes 0755 - - -
+d /var/sddm_themes/themes.work 0755 - - -