Is it possible to change the fontsize per user?

We get complaints from the shopfloor that, especially on mobile devices, the text is sometimes rather small for certain people.

I've tried to make a copy of the theme and change the fonts in there to a larger font.
After deploy I assigned the new theme to the specific user but it doesn't change the font size (and font size 20 is hard to miss I think ):
 

When I set the Default theme to Dark in the above picture and redeploy I do see that the Universal interface becomes dark mode.