Read by anything that must paint as app chrome while rendering inside a surface that flips mode — the
grid's own frame is drawn from within the grid, so it sets themeSurfaceProps('app', useAppColorScheme())
to select the app cell explicitly. The mode is not optional there: [mode] [surface] is a descendant
rule, and both the workbook root (app mode) and the sheet (grid mode) are ancestors, so CSS would pick by
source order rather than proximity. Stating the mode selects the compound [mode][surface] cell, which is
emitted last and wins.
The app chrome's resolved light/dark scheme — the sibling of
useGridColorScheme, before the Enable Dark Grid gate that one applies.