The gadget was altered several days ago. See Wikipedia:Dark_mode_(gadget). I needed to add
window.wpDarkModeAutoToggle = true;
to my common.js. I found its behaviour was a bit erratic for the first day or so after the change.

Note to check which script conflict with it.

Oh, is RedWarn. @Chlod can you please check this?

Seems to be caused by the toast bar. Should be fixed with commit a59ef311, deployed with Special:Diff/1070967907. I will note, however that RedWarn is very quirky under dark mode, and may not show text at all. That, however, is beyond my ability to fix without replacing the dialog system.

Understood. Thanks for your help!