Make menu colors customization easier for developers

Currently, the menu macro is using the navigation bar colors from the color theme .
However, it does not define its own color variables, and sets them by default to the colors of the navigation, it uses directly in the code the nav bar variables.
Thus, if a developer wants to intervene on the menu SSX and customize the colors of the menu, it's very hard to change these colors since they need to be changed in many places.

The improvement proposed by this issue is a refactoring of the Menu SSX in order to define menu own variables, initialized with the nav bar colors, but which can be changed easier.

The next step, that would need to be covered by a different task, would be to make it easier for users to customize the menu colors.