Anchor link and sticky menu with offset. How to do the same as in the example?

Question

css

Good afternoon! I came across a site that solved the problem of anchor links and a menu sticking on top. Here is an example boleznius.ru/%D0%B3%D0%B5%D1%80%D0%BF%D0%B5%D1%82%...
By clicking on any element from the content of the article, you are redirected to the subtitle of the article, but in this case, the subtitle is not at the very top of the page. For example, on any other site with a floating header, for example www.golovazdorova.ru , when you click on the table of contents, you are redirected and the header is not visible.
Question - how to make it so that it is indented from above using CSS / JS? Google did not help, plus in the example the transition is smoothly implemented.
I dug and dug in the code, but I didn’t find where the CSS is indicated, apparently it’s somewhere hereboleznius.ru/wp-content/themes/marafon-bol/js/scri... , but I don't know how to figure it out.