Easiest to just specify text and use the standard button appearance, but you can also choose a specific image. You assign a callback function for when it's clicked.
It's a small thing but I kept noticing it. I ended up making an iOS-style header bar in HTML, though I'll openly admit that it's not the greatest solution.
Instead of having to spend time styling the top bar and tab bar to look native, and battling with fixed positions and mobile browser inconsistencies, you just drop a line into your config.
Many people seem to think fixed position elements and dealing with scrolling in mobile web views is a pain: