It should use the dark wallpaper
Same thing in the Global Themes KCM. The preview image lives in the Breeze Dark global theme itself, moving the bug report there.
A possibly relevant merge request was started @ https://invent.kde.org/plasma/plasma-workspace/-/merge_requests/3941
Git commit 0ab8f61ad5ca0de221ae7f95dbf74a497bb47d36 by Fushan Wen. Committed on 20/02/2024 at 12:00. Pushed by fusionfuture into branch 'master'. lookandfeel: update dark preview for Plasma 6.0 FIXED-IN: 6.0 M +- -- lookandfeel/org.kde.breezedark/contents/previews/fullscreenpreview.jpg M +- -- lookandfeel/org.kde.breezedark/contents/previews/preview.png https://invent.kde.org/plasma/plasma-workspace/-/commit/0ab8f61ad5ca0de221ae7f95dbf74a497bb47d36
Git commit efa9aaf056e64b5bc1899933881a53f44a173e9c by Fushan Wen. Committed on 20/02/2024 at 13:20. Pushed by fusionfuture into branch 'Plasma/6.0'. lookandfeel: update dark preview for Plasma 6.0 FIXED-IN: 6.0 (cherry picked from commit 0ab8f61ad5ca0de221ae7f95dbf74a497bb47d36) M +- -- lookandfeel/org.kde.breezedark/contents/previews/fullscreenpreview.jpg M +- -- lookandfeel/org.kde.breezedark/contents/previews/preview.png https://invent.kde.org/plasma/plasma-workspace/-/commit/efa9aaf056e64b5bc1899933881a53f44a173e9c