TYPO3: Add special menu CE and add class="active"
<p>In <a href="https://en.wikipedia.org/wiki/TYPO3" rel="noreferrer">TYPO3</a> one can insert a special menu as a content element, you can select about eight different ways to collect the menu.</p>

<p>In the menu I selected there is no class assigned if the menu item is active, and I need to highlight it with CSS. And I'd like to know how to add a custom menu.</p>

<p>Note: Once I did apply the solution supplied in my own answer I realized that the main difficulty was (and generally is in TYPO3) the caching, be aware to refresh frequently otherwise you'll never be sure what is correct code and what isn't...</p>