Hi,
The plugin needs it’s own search form to function properly, unfortunately it cannot attach to existing search forms. It has to be styled separately.
Most styling you can do on the plugin back-end, under the Theme Options panel. I recommend trying out a few themes first, checking the preview window which one fits your case the best: https://i.imgur.com/eoXlTwB.png
Then changing the colors, borders, fonts etc..
