How to apply styling to Header columns of Readonly grid?

There's no direct way to do this or any other particular formatting for grid column headers, outside of the available parameters seen in the documentation. 

For what it's worth, this sounds more like a "design request" rather than a "requirement", since it probably doesn't actually prevent any functional requirements from being completed.

Thanks. Yes its a part of Usability design recommendation. i believe we don't have any workaround as well for the same.

I often find that people in charge of creating "requirements" need gentle yet constant reminders that design and requirements are separate things.

As a workaround you may put the grid within a card layout and put style over card layout that will be applied to grid too. This can take care of grid background but for font there is no way as of now! You can suggest this to the business if they like!

Thanks for the reply. Card layout style(background color) applied to entire grid not only to the grid header row.