The modified highlight plugin previously available here is no longer present as the changes I made have now been incorporated into the default MODx search highlight plugin, ver 1.4, available as part of the current AjaxSearch package that comes with MODx 1.0.0.
CSS tip
The search highlight plugin will highlight all text in the current MODx document that matches your search terms, by inserting a <span> element with class="highlight" around each term. This will happen even if the text is not part of the content being searched.
For example, consider someone searching for "linux bootloader". All instances of "linux" and "bootloader" will be highlighted. If you site is called "linux for beginners", with this site name appearing on every page, the word "linux" in the heading will be highlighted. If you have a block on every page titled "quotes on linux" the word "linux" will also be highlighted there.
On the one hand you may not regard this as a problem, but on the other hand it looks pretty silly when it is the site content/pagetitles/etc that are actually searched, not the site name or headings that are common to each and every page. One solution is to selectively target the <span> elements that the plugin inserts, e.g. instead of
.highlight { background: #FF0; }
you could give the content div/paragraph/whatever an id and use
#content .highlight { background: #FF0; }
or a class and use
.searched-content .highlight { background: #FF0; }