The only vaguely possible way I could address this would be if you provide the same image from each theme. If one of them does NOT show this effect I might be able to compare the code and suggest a correction to colors. You could also try the colors theme at a webtrees site to see if that has the problem.
Or, get your MAC technical guru to tell you / me what needs changing.
Alternatively I may need to add a warning to the downloads page that kiwitrees may not be suitable for use on MACs.
(ps – I deleted the previous discussion as it was unhelpful and confusing to others, wandered off into vague and irrelevant discussion about ipods/ipads etc, and never adequately addressed my requests for tests).