I will assume you know how to activate userChrome.css, so I will just provide some syntax:

This example is for just one tag (T_24label1), but you can make similar for T_24label2 through 5), This example has default color for selected message without tags as orange with white text and default for 'important' tag as red background with white text, and same color as untagged when selected.

/* ######  background color for messages that do NOT have tags   */
#threadTree tr.selected { background-color:    #ffa500 !important; color: #fff !important; font-style: italic !important; font-weight: bold !important;} 

/* label1 Important  */
#threadTree tr[data-properties~="T_24label1"] { background-color:    #ff0000 !important; color: #fff !important; }
#threadTree tr[data-properties~="T_24label1"].selected { background-color:    #ffa500 !important; color: #fff !important; font-style: italic !important; font-weight: bold !important; } 

A Reminder: This is not supported, and is subject to possibly not working in a future release. The syntax I provided, for example, was different in versions prior to 115.

Hi, David. Tks for you. Almost goi it all, but with 'working' tag i had some problem. I attach screenshots below, if you can to solve this i tell you additional tks :-)

You may have to dig into CSS to find that. I don't know what syntax is for custom tags.

The code for a custom tag named customtag is something like:

#threadTree tr[data-properties~="Tcustomtag"] {background-color:lightgrey !important;}
#threadTree tr[data-properties~="Tcustomtag"].selected {background-color:grey !important;}

https://support.mozilla.org/en-US/questions/1270814#answer-1278816

Tks for everyone! It can solve regrettable omission for me.