AFAIK, we can't change font color for SMS. I think Motorola developers were trying to make the SMS screen to appear same as other model, and as the results are SMS lag bug, font color can't be changed, and so does with the size.
I just can't understand why they don't make the screen just like the email screen. I mean, there's no problem with writing email, no lag, and text editor appearance adjusted by theme.
That's what make me think that E2 is just an experimental product for Motorola. But for an experimental product, E2 is just to sweet.