Remember, the code to create a link is
Thus, you're looking in the CSS for the a, a:hover, and a:visited selectors. These are between lines 40 and 52 in the Tumblr html.
<style>
body {
margin: 0;
padding: 20px;
background-color: green;
font-family: arial;
font-size: 0.75em;
color: blue;
}
a { //link as you see it on the page
color: black;
text-decoration: underline;
}
a:hover { //link when the mouse is hovering over it
color: white;
background: red;
}
a:visited{ //link after it is visited
color:blue;
}
<style>
You want to change the code in the bold section. You can use the color selector to refer to the text color, see here and here for more information on CSS font attributes.