Allow for configurable video border-radius

[mportiz08]

Bug/issue #, if applicable: 126292441

Summary

Allows for the border-radius to be controlled with a custom CSS property called --video-border-radius. The default is none (0px).

Testing

Steps:

1. Browse any content with video and verify that the border is unchanged by default
2. Use the inspector to configure a value for --video-border-radius and verify that the video corners are more rounded.

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• Added tests (CSS-only change)
• Ran npm test, and it succeeded
• Updated documentation if necessary