How to customize file upload button styles using css?

You can customize the file upload button styles by writing a custom css in the Form settings advanced option.

Step 1: Open your Google Forms > click on the addon icon > select Formfacade > Select Customize this form > Click on the Proceed button.
Step 2: In the Formfacade customize interface, click on the gear icon next to the form title.
Step 3: Form settings screen will be displayed. Select the advanced tab and add the custom CSS.
Step 4: You can change the background color, border, font, font color etc by applying the css styles to the "addfile" class as shown below.