Nav-unification: Selected submenu color is chosen by variable

[mreishus]

Changes proposed in this Pull Request

• When the nav unification flag is on, the expandable submenu will have customizable colors for its currently selected item. Previously, they were hardcoded to white fg and transparent bg.
  • This works fine for most themes, except light from wp-admin. It already has a white background so I need to be able to override the text cleanly.
  • Very similar to Nav-unification: Submenu colors are controlled by variable #47249, which covered unselected and hovered items. This adds selected (or active) items.

Testing instructions

• With nav unification on ( http://calypso.localhost:3000/?flags=nav-unification ), check the colors of the selected item in an expandable menu. It should look the same before and after.

[matticbot]

Test live: https://calypso.live/?branch=add/nav-unification-submenu-variables-2

[matticbot]

This PR does not affect the size of JS and CSS bundles shipped to the user's browser.

Generated by performance advisor bot at iscalypsofastyet.com.