The signature was #9a9a9a, picked against the dark ground. The app
switches the whole tree between lightTheme and darkTheme at App.tsx:25
and the 404 page sits inside that, so it also renders on the light
background - where #9a9a9a manages only 2.64:1 and reads as washed out.
#787878 is the balance point: 4.13:1 on the dark ground (#151518) and
4.14:1 on the light one (#F6F8F7), chosen by maximising the worse of
the two rather than tuning for either. Checked by rendering on both.