Color schemes: Add new modern color scheme option.

This PR adds a new color scheme option, which uses a high luminosity blue spot color, almost-black menu, and pure white for menu items. 
This helps increase contrast, and bring more consistency with some of the higher contrast colors used in the block editor.

Props joen, ibdz, shaunandrews.
Fixes #50504.

Built from https://develop.svn.wordpress.org/trunk@48277


git-svn-id: https://core.svn.wordpress.org/trunk@48046 1a063a9b-81f0-0310-95a4-ce76da25c4cd