You can customize the widget’s appearance by overriding its CSS classes.
The File Uploader is thoroughly annotated with CSS classes. That’s your starting point with deeper customization. You can find a class for every widget item by inspecting its elements or sifting through the widget source code.
Also, our widget is designed to organically inherit styles from your page: dialog elements get scaled in line with your font size.
Here are some topics that cover specific customization cases: