Forgive me if I overstep any bounds here, but I would like to share a cobbled-together userChrome.css script which successfully customizes the appearance of - in my case - Betterbird, but of course will/should work
for Thunderbird v115:

/* for folder pane */
#folderTree:focus-within [is="folder-tree-row"].selected > div.container {
background-color: forestgreen !important;
}

#folderTree:focus-within li.selected.unread > .container > .name, #folderTree:focus-within li.selected.new-messages > .container > .name {
color: limegreen !important;
}

/* for message list */
[is="tree-view-table-body"]:focus tr[is="thread-row"].selected, [is="tree-view-table-body"]:focus-within tr[is="thread-row"].selected, [is="tree-view-table-body"] tr[is="thread-row"].selected:focus-within {
background-color: forestgreen !important;
}

.unread > .container > .name,
.new-messages > .container > .name {
color: forestgreen !important;
font-weight: bold !important;
}

*|*:root {
--folderpane-unread-count-background: forestgreen !important;
--folderpane-unread-new-count-background: forestgreen !important;
}

#threadTree tr:hover[is="thread-row"][data-properties~="untagged"] {
background-color: limegreen !important; /* Highlight */
}

#folderPaneSplitter:hover {
background-color: forestgreen !important;
}

#messagePaneSplitter:hover {
background-color: forestgreen !important;
}


For yourselves, for different colors, simply choose the one(s) you want.
I chose shades of green since that is my fave color.

I give most of the credit to a person name "Morat" from the Mozilla
forum for much of the code, the rest I used Developer Tool in Betterbird
to parse what script needed to be changed and implemented in the .css file.

Hope this helps some folks who struggled to customize v115 where the
previous userChrome.css for v102 would not work anymore.

--
Revvie Quar

Gryphons ~RAWK!~

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