Update theme.css

[mrestrepo1]

Revised CSS to match Meta brand.

Motivation and Context

How Has This Been Tested

Types of changes

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

Checklist

• [ ] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have read the CONTRIBUTING document.
• [ ] I have completed my CLA (see CONTRIBUTING)
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.

[aclegg3]

Cool, can you add some images of your local build?

[dhruvbatra]

CC: @angelxuanchang, who has been interacting with Michelle.

[angelxuanchang]

Thanks Michele! @aclegg3 can you help review the PR? I'll DM you the visuals. In addition, I think there may be additional places where the fonts may need to be updated.