Ajax Search is a simple instant posts search widget.
Define your custom CSS rules in your theme's CSS file.
You can do it if you are a developer, the result template is in /ajax/simple-search.php I do not recommend modifying the plugin .
There exists lot of common issue like some posts have post thumbnail other posts don't have so the layout will screw up. If you have an idea how to solve it easily I will add to this plugin.