Twenty Seventeen: Add `.has-header-video` styles for custom color schemes.

Following [39413], this adds `.has-header-video` style definitions in
`/inc/custom-header.php` to support custom color schemes.

Props laurelfulford.
Fixes 38995.
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git-svn-id: https://core.svn.wordpress.org/trunk@39355 1a063a9b-81f0-0310-95a4-ce76da25c4cd