Can I use external libraries?

Definitely! Don't worry about a limit, you can use as many as you want as long as they’re adding value. Here are a couple of examples:

• Using dependencies to manage behaviors outside the scope of MUI (drag and drop, virtual lists, etc.) is great, but discourage replacing existing functionality (tooltips, modals, sliders, etc.). 
• Using dependencies to provide charting possibilities is great (Morris, Charts.js, etc).

Just be sure the license of the library you're developing with supports your use case and outlines any restrictions on the theme's page so buyers know what they're getting.