Thunderbird userChrome.css

userChrome.css

/* Tested on Thunderbird 115.12.2 (64-bit) */
/* Enable the preference toolkit.legacyUserProfileCustomizations.stylesheets in Settings -> Config Editor */
/* Create chrome folder in %APPDATA%\Roaming\Thunderbird\Profiles\[xxxxxxxx].default-release\ */
/* Create userChrome.css with below contents */

/* Change background color of unread messages to a light green */
#threadTree tbody [data-properties~="unread"] {
    background-color: #26655a !important;
}

/* Highlight Taged emails with background color rather than text*/
tr[is="thread-row"][data-properties~="tagged"] {
    background-color:
        color-mix(in srgb,var(--tag-color),#000 55%);
    color: var(--tag-contrast-color) !important;
    font-weight: bolder;
  }