Modal is not full width/height on mobile

[david-corneail]

Expected behavior
Modal is displaying full width or height on smaller screen sizes

Actual behavior
The Pharos modal is displaying spacing around the modal container and the browser window.

Steps to reproduce the issue

1. Step one: open up jstor.org on phone
2. Step two: select login from the header and launch the login modal
3. Step three: see how the spacing around the modal is causing layout issues. The expected behavior as that the modal takes up full screen to maximize the space for smaller screens.

Screenshots or code
If applicable, add screenshots or code to help explain the issue.