Move adapt-vimeo and adapt-youtube theme LESS styling to Vanilla plugin

[guywillis]

Subject of the issue

The theme styling from the adapt-vimeo and adapt-youtube plugins should probably be moved from their respective plugins to Vanilla for consistency.

Your environment

• Master FW & AT

[kirsty-hames]

The transcript styles should also be updated to include expanded/completion styling as per Media. Or perhaps a mixin could be created for shared styles across the plugins as these are typically styled the same and currently duplicated code. We did similar for the Notify button.