desktops: split light/dark wallpapers in gnome + bianbu postinsts

[igorpecovnik]

Summary

The Armbian wallpaper does not show up on Ubuntu resolute (GNOME 47+) GNOME builds, while it does on noble (GNOME 46). Cause: GNOME 42+ added picture-uri-dark for org.gnome.desktop.background. When the session runs in dark appearance (default on resolute, and the default on Bianbu too), the shell reads picture-uri-dark, ignores picture-uri, and falls back to the schema default if picture-uri-dark is unset. The Armbian dconf seeds in gnome.sh and bianbu.sh only set picture-uri.

Noble's GNOME 46 happens to fall back to picture-uri more leniently, which is why the regression only surfaced once resolute landed.

Fix

In both gnome.sh and bianbu.sh, set both keys to the matching variants Armbian already ships under /usr/share/backgrounds/armbian/:

• picture-uri → armbian18-Dre0x-Minum-light-3840x2160.jpg
• picture-uri-dark → armbian03-Dre0x-Minum-dark-3840x2160.jpg

GNOME tracks the user's appearance setting and picks the matching one. The screensaver block doesn't change — org.gnome.desktop.screensaver has no -dark variant; the dark image stays the right default for the lock screen.

Test plan

• Boot a fresh resolute/GNOME image. Default appearance is dark → dark wallpaper renders. Switch to light appearance → light wallpaper renders.
• Boot a fresh noble/GNOME image. Wallpaper still tracks the appearance setting; no regression.
• Boot a fresh Bianbu image (K1). Same expected behaviour — dark by default, swap to light works.
• Live test on a running system without rebuilding (per-user, instant):
  gsettings set org.gnome.desktop.background picture-uri \ 'file:///usr/share/backgrounds/armbian/armbian18-Dre0x-Minum-light-3840x2160.jpg' gsettings set org.gnome.desktop.background picture-uri-dark \ 'file:///usr/share/backgrounds/armbian/armbian03-Dre0x-Minum-dark-3840x2160.jpg'

[coderabbitai]

Walkthrough

This change modifies the GNOME post-installation configuration script to write an additional dconf setting. A new picture-uri-dark key is appended to the GNOME background configuration block in /etc/dconf/db/local.d/00-bg, pointing to the same Armbian background image as the existing picture-uri setting. This enables the dark theme variant of the desktop background.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title accurately captures the main change: adding support for light/dark wallpapers in GNOME and Bianbu postinstall scripts by setting picture-uri-dark alongside picture-uri.
Description check	✅ Passed	The PR description clearly explains the problem (GNOME 42+ added picture-uri-dark causing wallpaper not to display on resolute), the root cause (dark appearance mode reads picture-uri-dark instead of picture-uri), and the specific fix (adding picture-uri-dark to the dconf configuration).

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch gnome-dark-wallpaper-resolute

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