I'm using the stylish addon and it works great. However I would like to substitute a specific font - say Arial - with another of my choice every time it is found instead of setting a global font (which I don't like as I just want to substitute Arial) or using a stylish rule for every page. Is it possible to do this? Maybe with a greasemonkey script?
You'd need greasemonkey, not just stylish. The script would have to look at computed styles for every element, and account for the fact that Arial can be accessed by several names.
Another angle is to reconfigure Arial to be replaced by another font. This is more foolproof and less resource intensive.
Assuming you're on linux, put this in
I follow this, and then set
in a Stylist addons.