Hi there Stephen! Thanks for getting in touch and welcome to the forums. 🙂
Good question. The main calendar is a custom post type archive rather than a standard page template, so it could be possible that your theme’s main menu isn’t accounting for that.
You might be able to target that specific menu item in the CSS with something like this in your style.css file:
.events-archive .menu-item-49 a {
color: #39ad36;
}
However, the best route here might be to connect with the theme author and ask for a better method to add the .current-menu-item class to the item automatically instead. 🙂
Cheers!
Geoff