1 - Compose area background -- I assume you're running on Android 3.0 and above. Yes, the appearance of edit controls has changed, and they now use the same background as the overall theme (dark/light). Android 2.* always used white background for text edit fields.
2 - Being able to show a "list style" preview of new messages is a new feature of Android 4.1, my app makes use of it. Not sure why you'd want it off -- is it doing something bad, unexpected?
FWIW, you can enable "privacy" notifications (in the app's settings, in Extended mode (top item), in the notifications section), then the app will only show "X unread messages".
3 - You can adjust the font size using the device's hardware volume keys. Works in message list, message view, message compose modes (not in the account list yet).
4 - New content in outgoing messages in plan text only at this time. What you see on the device when editing is using whatever font Android has for "just text". On Android 4.*, it's Roboto, a sans-serif font.