It's good that the CSS is customizable, but many of your clients won't customize it, and the default text size is too small. I did a quick test of increasing comment bodies from 13px to 16px, and comment headers from 12px to 15px, and it made everything much more pleasant.

Consider increasing the default font sizes? If comments are worth writing and reading, it's worthwhile making them eye-friendly and easy to read.

Thanks