Unfortunately the developer doesn’t seem to support the theme; a pity, because I find this the one huge weakness of what is otherwise a very nice theme. Personally, the problem with the light text is one of the big issue keeping me from upgrading to paid on this one.
Anyway, you can – sort of – fix the issue by inserting the following lines in custom CSS in your Layout settings:
.entry-content p, li {
color: #000;
}
Depending on what text elements you use, you might need to add some more tags. It’s not a perfect solution, unfortunately, because the fonts being used in this theme don’t really look all that good in solid black, but it’s still better than the difficult to read light gray.