The font is unreadable because it is likely set in a media query. I would suggest looking at the mobile site in the browser then inspect the element so you know what CSS element it is set with.
Then you have a couple options.
Write a new media query that over rides the old one in the CSS editor, if the theme creator hasn’t disabled it or that your permissions disallow it. You’ll find it under Appearance. You man need to recreate the entire media query though. I don’t like doing this because it’s always been a bit buggy and one theme update could make you have to start over.
You can download the theme and edit the CSS files directly, however the next update will erase your changes.
I would suggest you create a child theme with your own CSS files. There are plugins that will help you copy the files so you can edit them.
These suggestions are for Wordpress direct install, if you’re using dot com you’ll need to go to their support.
If you need help with how to do do any of that, well, there are some courses you can take on CSS here, or I would suggest you go to wordpress.org and post directly on their forums. This site isn’t for Wordpress questions, this is for the Free Code Camp courses.