Can't have larger TWebPopupMenu without transparent background

Using TMS Web Core with Delphi 11.2.

I need a popup menu with larger font. So I use a TWebPopupMenu control and introduced a css class for that in the .html file. In this class, I set the "font-size", "color" and "background-color" properties. The font size is now OK, and also the color. But the background color always stays transparent (the control doesn't react to the setting).

So my question: How can I get a popup menu with a larger font but with a colored background?

This CSS will set the background-color

      .drop-label  {background-color:red;}
      .drop-label:hover  {background-color:blue;}

That works - thank you!