You can add icons to src/icons and run gulp build-icons. This will make the icons available for use in the contextmenu using the icon property.
So for example the file checkmark.svg wil result in the CSS context-menu-icon-checkmark which you can use by using the icon option when defining a menu item.
var items = {
firstCommand: {
name: "Paste",
icon: "checkmark" // Class context-menu-icon-checkmark is used on the menu item. This is generated from checkmark.svg
}
}
You can use the _variables.scss to adjust variables on pretty much everything you want to change.