As you noted, the issue is going to be more with your iPad display and glare, and not really with FM themes. To try and combat this with themes you need to think about using high constrast design.
There's useful info here:
Particularly I like the R&D behind solarized:
There are other good tips in high contrast development as well, IE Larger bolder fonts, less text.
Overall, the ability to save theme changes globally across your file will definitely come in handy.
Now if only there was a way to read from the native light sensor as part of your navigation scripts!
Thanx for this, I will have a look.
You should definitely blog about your findings!
One thing to consider about the themes is a lot of themes will you a shade of gray or a lighter shade of a color for the fonts. Also the field color might not be white. If you find a theme you like, I would recommend changing the font color to the darkest shade of that color.