The easiest way to add some CSS and JS:
JS
String customJs = """
function saveEntry() {
run("add.groovy", \$("#title").val(), \$("#text").val());
}
""";
HTML.js(customJs);
// Or:
System.out.println("<script>" + customJs + "</script>");
(please remember to use "$" instead of just "$" inside of Strings)
CSS
String customCss = """
.note {
border: 2px solid #bbb; padding: 0.5em 1em; margin-top: 0.3em; width: 50%;
}
""";
HTML.css(customCss);
Html
More details on adding HTML: Script Output