If you take a look at the source tree for Liferea there's a file, css/liferea.css, which contains the customization that you're asking about.
There's a section of this file:
Color Definitions: ================== To allow using GTK theme colors the following key words will be replaced with the corresponding GTK theme color values: ... ... GTK-COLOR-TEXT GTK-COLOR-NORMAL-LINK GTK-COLOR-VISITED-LINK */ body { background: #GTK-COLOR-BASE; color: #GTK-COLOR-TEXT; padding:0; margin:0; } blockquote { border-left: 2px solid #GTK-COLOR-DARK; background: #GTK-COLOR-BG; padding: 5px; font-style: italic; margin: 5px 20px; clear:both; } a { color: #GTK-COLOR-NORMAL-LINK; } a:visited { color: #GTK-COLOR-VISITED-LINK; }
Within this file you could change the colors for the a and a:visited sections, overriding these settings to what ever color you desire.