To change the style directly from a CSS file, it is recommended to only go through classes instead of hard-coding it.

<?php

    $functions = array("default", "html", "keyword", "string", "comment");

    foreach ($functions as $value) {
        ini_set("highlight.$value", "highlight-$value;");
    }

?>

And in the CSS file (for example) :

.highlight-html { color: #000000; }
.highlight-default { color: #0000bb; }
.highlight-keyword { color: #007700; font-weight: bold; }
.highlight-string { color: #dd0000; }
.highlight-comment { color: #ff8000; }