See https://support.mozilla.org/en-US/questions/1464739#answer-1675906

If you want a solid colour, replace the background-image part with something like:

background-color: lightgrey !important;

I already use "background-color" (please see my first post).

I tried to use "background-image" instead "background-color", but that does not work at all (maybe because I use the KDE Plasma desktop environment?).

Using:

/* Odd rows color */

1. threadTree tr .card-container {

 background-color: #27272a !important;

} /* Even rows color */

1. threadTree tr:nth-child(2n) .card-container {

 background-color: #2d3237 !important;

}

works fine, just missing the "top" transparent marking with system color for selected item (there is only border color). Please see attached screens in my first post.

With v115, the "card-container" parameter was not used and everything worked fine (stripes with transparent markings in system color for the selected item).

I am sorry, posting replaces "#" for "1. "

To set the background colour for selected cards, you can try this:

/* card bg colour for selected messages */
tr[is="thread-card"].selected .card-container {
     background-color: lightgreen !important; 
     outline: 1px solid red !important;
}

Replace lightgreen with transparent if desired.

Thank you, that solved my problem.

For completeness I attache my settings.