https://searchfox.org/mozilla-central/rev/6a41d8d7ad42d57cf1adcc7067fd8f1689c42ab3/layout/style/PreferenceSheet.cpp#44 currently enforces that chrome docshells don't get colours adjusted in HCM. I think instead of relying on the type of docshell, we should move to depending on the origin of the document, and adjust colours everywhere except chrome:, resource: and about: documents (except about:blank, about:srcdoc, about:printpreview and one or two other exceptions like that).

To make that possible, we'd need to review what in-content pages like the prefs and the network error pages and the like look like in that case. We can use the prefers-contrast and/or forced-colors media queries to ensure that they use system colours in HCM.
We'd probably want to put the change behind a pref so we have time to adjust CSS.

Oops, wrong bug.