The HtmlReporter explicitly generates css classes together with the html elements, based upon css styling re-used from the Gradle JUnit plugin.

Stylesheets, a minimized version of JQuery javascript library plus some icons are copied at report-generation time from the jar-file to the report output directory.

Styling the back-to-top arror/button is done as a combination of JavaScript plus some css styling, as described in http://www.webtipblog.com/adding-scroll-top-button-website/.