Create your own down arrow image and set the menu attribute:
StaticPopOutImageUrl="~/Images/your-drop-arrow.gif"
For the dynamic dropdown menu items themselves, they'd still use the default
side arrows, but since any additional levels of flyouts are to the side, it
makes sense. If you want to change that, add your own DynamicPopOutImageURL.
For the simple case, make a .gif with a black arrow and the background set
to transparent so that menu highlight colors will show through it.