how do you define an extra key e.g.
"highlight": "var(red)",

under your `globals`.

that appears to be missing in the default Monokai color scheme config?

I have this in my Preferences.sublime-settings--User

{
"color_scheme": "Packages/Color Scheme - Default/Monokai.sublime-color-scheme",
"highlight": "var(red)",
"theme": "Adaptive.sublime-theme"
}