It looks like your text is already set to #000 (black) which is the darkest it can get, but what’s making it look hard to see could just be the font-weight. The font-weight is set to 200….so you could increase that to 300, 400, 500, 600, or 700 to see if you like that a little better. This is near the top of the css file around line 160 –
body {
background: #fff;
border-top: 2px solid #ff7632;
color: #000;
font-family: 'Source Sans Pro', "Helvetica Neue", Helvetica, Arial, sans-serif;
font-size: 20px;
font-weight: 200;
line-height: 1.5;
}