how to make a menu sticky

This depends on the theme. The “Hyrdra” and “Evolv” themes come with a sticky navigation menu in the header by default.

There is no way to add sticky behavior to a menu from the Customizer. However, we’d encourage you to submit a feature request for it if that is something you’d like to see in a later version.

Also, since the BoldGrid themes are basically WordPress themes, they can be augmented via the child theme system. Alternatively, you could use browser tools to identify the CSS selectors necessary to add a sticky navbar effect.