What I wanted to know whether it was possible to change the font just from Settings in Jira. I couldn't find anything like that. I used to use Stylish plugin before Google banned it from their store for privacy issues. I think it should be possible to have a workaround w/o any plugin somehow, but I haven't looked deeper into this.
However if there is no option for custom font in Jira GUI, it would be nice feature to have for these cases.