Styling

You need no CSS. Sub-menu pointer-events and the dropdown caret rotation ship as inline style, so the components render correctly with zero CSS on any MUI version.

Per-item styling

Style a single item with :sx (MUI v5+) or :style on its item map:

{:label "Delete"
 :left-icon (icon :trash)
 :sx {:color "#f78166"}}

Class hooks

For theme-wide rules, target these namespaced classes:

Class	Element
.jnm-menu	every Menu the library renders
.jnm-submenu	nested sub-menu Menu elements
.jnm-menu-item	an item that opens a sub-menu
.jnm-caret, .jnm-caret-expanded	the dropdown caret

Overriding the caret rotation through a class needs !important, because the rotation is inline.