First the icon parent class may be different from theme to theme, open webpage in a browser, right click the icon and inspect the element:

note the parent class is wcmenucart-contents for me, so i can style it below with your fontawesome code:

(find the code here https://fontawesome.com/cheatsheet?from=io):

.wcmenucart-contents .fa-opencart:before {
content: “\f290”;
}