I had a quick look at the Global style settings and don’t see this style option listed for the filename boxes so we may need to write one of our own and insert it in the Custom CSS box.
Do you have a link to the form? So I can take a look at the CSS style associated with the filenames.
In admin mode the classname is called .qfb-edit-file-upload-inner
And that is what you would target when writing a custom style to change the default color, border and text color etc
I am guessing on the front-end the class name may omit the -edit- part of the class label. But can’t be sure until I see a form with the filename field box visible.
What styling did you want to achieve? Drop the green background? Or change text color too?