Header media bg color fallback

[cjg89]

Description

Enforces a default background color of #797979 on .header-media when no .bg-* modifier classes are also present on the element.

Motivation and Context

Helps ensure that black and white text within media headers is readable when a header video/image fails to load.

See also: https://github.com/UCF/Main-Site-Theme/pull/166

How Has This Been Tested?

Reviewable in Dev.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires an update to the documentation.
• [ ] I have updated the documentation accordingly.