From the Mozilla Bugzilla entry:
This patch adds the platform agnostic media selector and changes the way our themes behave as follows: If the default Firefox theme is selected, Firefox will match the system appearance (current default theme in light mode, dark theme in dark mode). Note that about:addons will continue to show “default” as the selected theme, even when it is technically using the dark theme under the hood to match the system’s dark mode. If any Firefox theme other than “default” is selected in about:addons, Firefox will not change themes when the system appearance changes.
This is missed in the release notes. I think this is true for macOS and Windows. I am not sure about other platforms.