[ ] for each page and width, the look service (src/looks/service.ts) takes a second picture with the dark theme forced (src/design/themes.ts), and stores it in its own slot so the light and dark pictures never collide
[ ] older looks without a theme still read as light
[ ] test: one preview look records light and dark for a page; the slots differ
Files: src/looks/service.ts, src/core/looks.ts, one migration if a column is needed.
Part of the former #2708.
Done:
Files: src/looks/service.ts, src/core/looks.ts, one migration if a column is needed.