While much of the conversation here is about the userstyle.css I was trying to change the font in the rest of the UI. I suspect my use of the AppImage on linux might be preventing my global settings. At any rate, the following snippet in userchrom.css finally got me the size increase I wanted.
html *
{
font-size: 13pt !important;
}
!important being the important part. the puns 
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