The Themes is an add-on, not a built-in. [So auto-detect resolution and auto-scale fontsize is off the table.]
But I set the fontsize to 20pts and it worked well.
Gramps also remembers window placements & resizings. (They are stored in the gramps.ini file in your User Directory.)
So there’s a viable workaround to any problem with too tiny fonts on high-res screens… and it just requires installing an add-on with zero new development.