I'm using the revision library here to specifically allow for reverse compatibility of parsed Preferences (that's the test that is currently failing). Try leaving theme but making its derive tag #[revision(start = 1, end = 2)], then the new light and dark ones #[revision(start = 2)] (and changing revision to 2 at the top of the Preferences struct). Once you get the tests passing, go to Redlib in an incognito browser and export a new Preferences string (with a revision of 2) and add it to the KNOWN_GOOD_CONFIGS array.

Is there any way we can preserve this behavior so someone with the REDLIB_DEFAULT_THEME env var set doesn't have to change it? Possibly by giving default_theme_light and default_theme_dark an alias of REDLIB_DEFAULT_THEME in order to make it the default across both?