1. Log in to Short.io.
2. Shorten a link. Click on 'Password protection'

If the link is already created, open the page for editing.

3. Click on the "Password protection" tab.

4. Click Customize CSS

5. Scroll down.
6. Add CSS.

Here're the examples on how to override CSS styles: http://help.short.io/en/articles/5015540-custom-css-examples-for-password-page