Fix ImageColourSchemeSwitch ignoring OS dark scheme in system mode - #337

Issue

ImageColourSchemeSwitch (and so Logo) shows the light image when the OS is in dark mode and the user hasn't picked a mode.

It decides on useColorScheme().mode, which stays "system" until a mode is chosen, so mode === "dark" is false even when the page renders dark. Seen in an app on 0.7.2-rc.0: the page had data-mode="dark", but the light logo sat on a dark header.

This seems to be the case since v0.6.0.

Fix

• Use the resolved colorScheme instead of mode
• Base data-testid on the same value
• Add a regression test for system mode with OS dark preference
• Add a changelog entry

ColourSchemeButton is the only other user of useColorScheme, and it already handles "system".

Consumer note

Apps passing fixedTone as a workaround can remove it once this is released.