Where can I find to modify the css code of the menu?

Joomla plugins\nextendmenutheme\[theme name]\[theme name]\style.less
WordPress wp-content\plugins\nextend-accordion-menu\themes\[theme name]\[theme name]\style.less

You can use css codes in less files the same way, as in the css files, the only difference is, that in less files there are variables, and the selectors can be specified in different ways too.