Lesson 1: Don't forget about the Light theme.
I developed the app completely using the Dark theme on my phone and the emulator. Lo and behold switch it to Light and some parts of it were unreadable, especially the ApplicationBar. My first submission was rejected because of this. Other parts just didn't all that good on the Light theme. We'll see how swing #2 goes...
Lesson 2: Don't set the ApplicationBar colors unless you really want those colors on both themes.
You can't DataBind them and they will be switched automatically per Theme by WP7, but not if you have set the colors by hand in your XAML.
Lesson 3: Don't use color icons on the ApplicationBar
Again, they may look great on the dark theme but switch to the Light theme and they will be drawn as black outlines at best, black blobs at worst. Use White on Transparent icons. They look fine on both themes. There are some good ones online. Don't include the outer circle (the app bar adds it) and set the image itself to 24x24.