this is being worked. The next release will have a slightly bigger font size and in the new theme it will all be handled in a far cleaner way with a much more appropirate size font. The problem is the current default theme is very old and the code is very volitale. It was using em units which componed font-sizes resulting in very small font-sizes.
First the spec has changed for px in all browsers IE 9 and up so that its scaleable.
no em is not relative to base font size that is rem. Em is a relative unit to the parent element based on the base font size. So if you set the body to 10px font size then set div.main to 1.6em its 16px but if you then set another div.section in div.main 1.2em would be (16*1.2) not (10 * 1.2) then set a button in that div to 1em in div.section its equal to (16 * 1).
Over years and a large spageti codebase this grows exponentially out of control...you end up with (10*1.6*0.5*2.1*0.9*0.8*0.7*1.2*1.*0.9...)
Also changing the font-size forum wide is a horible practice! just youe browser scale + -
In the new theme we are using px for development and then have a build step to convert it all to rems for production but we are also using standard componet sizing based on material design laguage font-sizes
the new theme is far away unfortunately probably not this year but as I stated, I did introduce a fix to prosilver which may make it into 3.3 potentially which should get out this year.