Change Font on Text Box

My site uses the Uglyqua font but this font does not display an underscore on the screen when the underscore button is pressed on the keyboard. It’s causing my customers issues as my Extra Product Options text box is compulsory & asks for an Insta handle & a lot of people have underscores in their Insta handles.
Is there a way I can change the font just for the text that is entered in the Extra Product Options text box?
In the FAQs for this plugin it states: “Style the fields to match with your store: You can style input fields, labels or the container itself(we call it a wrapper) by associating a CSS class from your stylesheets. Simply add the class name to input class, label class or wrapper class to style them individually.”
However I’m really not sure what that means, i.e. what code to write and where it should be included.
Would appreciate any help anyone can give me!

The page I need help with: [log in to see the link]