Thanks for that link. I'll have to subscribe to Reddit and take a look at the discussion.

Sorry: question posted in error. Apologies.

I'm happy to report that I'm almost there, with the layout and most of the colour choices now working in version 127.0 running on Windows 10.

There's one thing I can't sort out: I'd like to have a tab change background colour when hovered over. This is the code, inherited from the previous PC:

[hash] TabsToolbar:not(:-moz-lwtheme) { [hash] tabbrowser-tab:not([selected="true"]):hover [hash] tab-background

  background-color: red !important;

}

I don't know if giving that in isolation is actually helpful or not, since it could be being overridden by something else that I'm not recognising, but just in case the problem is in these lines I thought it might be worth posting them. Am I overlooking something obvious?

Many thanks.

#TabsToolbar:not(:-moz-lwtheme) {
#tabbrowser-tab:not([selected="true"]):hover
#tab-background
   background-color: red !important;
}

That isn't legal CSS. I think they meant for it to be this:

#TabsToolbar:not(:-moz-lwtheme) #tabbrowser-tab:not([selected="true"]):hover #tab-background {
   background-color: red !important;
}