What CSS file is this about ?

Do you mean styling the Firefox user interface via userChrome.css ?

• https://www.userchrome.org/what-is-userchrome-css.html
• https://www.userchrome.org/how-create-userchrome-css.html

Exactly: I have a userChrome file with the following code:

/* FIREFOX CSS +/


/* global font */
{
  font-size: 12pt !important;
  font-family: Liberation sans !important;
}

#main-menubar menupopup {
  -moz-appearance: none !important;
  background-color: #2F4F4F !important;
  color: white !important;
}

menu[disabled="true"],
menuitem[disabled="true"],
menucaption[disabled="true"]
{
color: #C0C0C0 !important;
}

/* arrow right if needed */
#toolbar-menubar .menu-right {
filter: invert(100%) !important;
}

/*bookmark popup toolbar background */
#navigator-toolbox > toolbar:not(#TabsToolbar):not(#toolbar-menubar) menupopup[placespopup]
{
  -moz-appearance: none !important;
  background-color: #000000 !important;
  color: white !important;
  border: none !important; /* optional */
}

/* links */
#navigator-toolbox > toolbar:not(#TabsToolbar):not(#toolbar-menubar) :-moz-any(menuitem.bookmark-item, menu.bookmark-item) 
{
  color: white !important;
}

/* arrow right if needed */
#navigator-toolbox > toolbar:not(#TabsToolbar):not(#toolbar-menubar) .menu-right 
{
 filter: invert(100%) !important;
}

/* Tweak the separators in all menus */

/*menuseparator*/ 
{
-moz-appearance: none !important;
list-style-image: none !important;
background: none !important;
border-top: solid 2px #CCCCFF !important;
border-right: none !important;
border-bottom: none !important;
border-left: none !important;
padding: 0px !important; margin: 0px 0px 0px 27px !important; -moz-appearance: none !important;
}

Do you have any of the 'proton' entries in about:config set to false? You should change them to true.

Note that your first rule doesn't have a selector (there would at least be a '*' wildcard).

/* global font */
* {
  font-size: 12pt !important;
  font-family: Liberation sans !important;
}

Hi.

TerryN21: tried that and it did not change anything.

cor-el: Thanks. Implemented the 'wildcard' star. Nothing changed. Do you see any obvious errors in the selector naming etc?

Maybe I should explain what I actually see.

It seems the userChrome.css is active and working in as far as

1. The Menu dropdown now has a white background colour and black text but not teal background and white text.

2. The Bookmark dropdown is totally white except for the small icons for particular websites. So both background and text colour is white.

Haagen