Try this code in userChrome.css:

menupopup > :is(menuitem, menu) {
/* color of the text */
color: yellow !important;
}

Hi,

Thanks for your suggestion, but it is not the text color I want to change it is the hover colour (shown in grey in the attachments) that I want to change.

Generally speaking, for a given selector:

some-selector:hover { background-color: yellow !important; }

If cor-el's CSS worked for the color, then:

menupopup > :is(menuitem, menu):hover {
  background-color: yellow !important;
}

Probably you already looked up the userChrome.css part?

Disclaimer: it's community-supported rather than officially supported.

If not, I have a site to get you started: https://www.userchrome.org/

jscher2000,

Thank you, that was it.

Yellow was too bright, gone instead for lightskyblue, which goes well with my red theme.

Thanks again.