[Action-N]

Fasted way would be edit the style variable, however I don't see where else it'd be used on your site to know what it'd look like there.

Go to Style Variable Editor > Comments > postbitlite_header_color an change it to the white font for your black background theme an black for the two white themes. Might even get away with saving them with no color an see if they inherit the main font color.

Alternative would be edit the css for each theme, find:

.commentbit_head {color: {vb:stylevar postbitlite_header_color}; text-align:{vb:stylevar left};margin-left: 90px;}

and change '{vb:stylevar postbitlite_header_color}' to the color you need.