How many times were you able to reproduce it?
Steps to reproduce
- Go to https://en.wikipedia.org/api/rest_v1/page/mobile-html/Dog in the browser
- Open console
- Paste pagelib.ThemeTransform.setTheme(document, pagelib.ThemeTransform.THEME.BLACK) in the console to change theme to black
Body is black
Body is white
It appears that this has to do with the recent fix for T214728 that unsets .content background color.
In mobile-html pages content applies to body so setting unset or initial on content overrides background from pagelib_theme_black_body. Turning off those 2 content rules fixes it.