debian/patches: Add 0001_guest-session-Make-path-to-lightdm-guest-session-exe.patch. Allow providing a fixed path to the lightdm-guest-session executable at build time. (Relates to #986818).

debian/rules: Enforce a specifc path for the lightdm-guest-session helper executable (shipped in lightdm bin:pkg). (Closes: #986818).

debian/9*-arctica-greeter-*.conf: Adjust to new LightDM section names. The [SeatDefaults] section is deprecated with Debian 11's LightDM version.

debian/arctica-greeter-theme-debian.install: Move all logo images into the default a-g-theme-debian bin:pkg.

debian/30_arctica-greeter-theme-debian-futureprototype.gschema.override: Fix typo in SVG background image path. (Closes: #924789).

debian/arctica-greeter{,-guest-session}.install: Build installs executable helper scripts to /usr/libexec. Handle that appropriately.

debian/{control,compat}: Drop compat file. Switch to debhelper-compat notation in control file. Bump to DH compat level version 12.

debian/*: Artwork update for Debian buster. Allow more flexible installation of Debian-specific artwork on Debian buster and Debian stretch(-backports).

debian/rules: Update DH overrides: Introduce dh_missing override, drop dh_auto_build override (was doing nothing), drop theme logo copying.