“Hard” (high contrast) should use the bg0_h value for the background: #f9f5d7 or rgb(249,245,215).
This is a light “off white”. If you have blue blocking mode or a night color mode enabled, the yellowing effect will be exaggerated.
For what is worth, I don’t think gruvbox makes a good light theme because it’s pretty low contrast especially compared to a lot of more recent themes. I say this as a die hard gruvbox dark user.












Material themes definitely tend to be softer. I hate them as a rule.
Gruvbox material fg0 definition
Edit:
Surprisingly, this passes a basic accessibility check. https://webaim.org/resources/contrastchecker/?fcolor=654735&bcolor=F9F5D7