Comments on your Extra Theme blog get their text color from the global text color set in the Theme Customizer (under “General Settings > Typography Settings”). If you wish to change the text color just for comments,  e.g. to fit in with your design, you can easily do so with the following CSS:

/* Change the comment text color */
.comment-content { color: #333 !important; }

The above sets it to a dark grey – just change the #333 part to the name or hex code of the color you wish to use.

You can add this to the theme by pasting it into the “Custom CSS” box found in the ePanel at “Extra > Theme Options > General Settings” (at the very bottom).