Does Horde_Text_Differ_Renderer have built-in methods to format the text it outputs?
ie. colors, apply css classes? etc?
As you can in the source, Horde just add prefix to lines it sees differences in.
For example, for added line, it will use :
protected function _added($lines) {
return $this->_lines($lines, '> '); // prefix with >
}
So you can modify this class to add whatever you need here, or better, you can override it with your own class modifying only the methods you need to change, leaving the rest untouched.