cool that you got it working...something I just thought of, if FBReader is ever updated with a firmware update, it might overwrite the changes you made. So maybe keeping a backup is not a bad idea, especially if you dig deep into a custom style.xml file.
I about the suggestion to just copy the styles.xml from another linux machine...one caveat I can think of is how the version on the 360 would handle color settings. Of course no reason you could not just setup your other reading device to look the way you want on the 360, grab the new style.xml and copy it over to the 360. I am just wondering if that would be faster then the trial and error manually editing the file. Still some trial and error would exist but at least it could be quick changes.
As for the Windows version, you know, the other night I thought about that and yup they decided to not stick to the same design as on other platforms.