[help-wanted/beginner friendly] Option for dark menu item font

[scottshapiro]

Issue: On desktop when the header image is light or white, the menu items appear camouflaged.

Request: Automatically recognize whether the header image is dark or light so that the menu items appear with contrast

[lacymorrow]

Take this into account: http://leaverou.github.io/contrast-ratio/

[lacymorrow]

Still an issue. Not exactly implemented in Ghost but still seems relevant to implement... I #wontfix but will accept PR for this.