Default How do custom ROMs handle buttons?

The kindle Fire HD does not have any buttons on the front side. If you're running an app in fullscreen then a little picker shows up at the bottom of the screen that contains the buttons "Back, Home" etc.

How do other ROMs handle this lack of buttons? I would like to install CyanogenMod on my Kindle, but I am not sure how it will cope with the Kindle having no buttons.