He's teaching you that reading white text on a dark background is actually harder to do and strains the eye more.  Consider his comment your education on the usability/readability of different contrast ratios (a pretty well-reasoned post on the topic here: <a href="http://www.456bereastreet.com/archive/200608/light_text_on_dark_background_vs_readability/" rel="nofollow">http://www.456bereastreet.com/archive/200608/light_text_on_d...</a> ).  And consider my comment your education on how your tone can be annoyingly preachy and smug.