A large portion of your blog audience read your blog through a feed reader. If you’re using a theme with a light colored font (such as Roller’s brushedmetal theme), it looks great against the background of your blog, but barely visible against the background of some feed readers.
Light colored font against the blog’s contrasting background:
Light colored font against a feed reader’s light background:
On the other hand, I wonder why feed readers don’t convert the font color to something that renders well against their background. I suspect some of them do, but bloglines doesn’t. The moral of the story is you simply can’t rely on them doing this for you.