Use Inline Styles(CSS)

<BubbleMenu editor={editor}>
  <div
    style={{ backgroundColor: 'white' }}
    className={cn(
      "border border-external-black-10 shadow-sm rounded-md px-2 py-1 flex gap-1",
      toolbarClassName
    )}
  >
    <Toolbar editor={editor} />
  </div>
</BubbleMenu>