in the previous versions of moodle v2.7.x and v2.8.x that I used in the past, I could add a custom CSS code on top of a moodle page. Now I have moodle v3.8.2 and this way does not work any more. I enabled Administration->Plugins->Blocks->HTML->Allow additional CSS classes.
I would like to add the double underline mark via HTML.
border-bottom: 1px solid #000;}