Thunderbird

UI Customization

Enable user content/chrome

Settings → General → scroll to bottom or use search → Config Editor...

toolkit.legacyUserProfileCustomizations.stylesheets → true

In the profile directory create chrome directory and userChrome.css file.

Dark theme with less contrast

Text background color (both reader and composer, has to be set in settings): #303841.

userChrome.css

:root {
    --main-bgcolor: #303841;
    --foldertree-background: var(--main-bgcolor) !important;
    --toolbar-bgcolor: var(--main-bgcolor) !important;
}

#threadPane > tree-view,
table[is="tree-view-table"],
#messagePane {
    background-color: #363e48 !important;
}

#folderPane,
#accountCentral,
#messageHeader,
#attachmentBar,
th[is="tree-view-table-header-cell"],
th[is="tree-view-table-column-picker"] {
    background-color: var(--main-bgcolor) !important;
}

#status-bar {
    background-color: #2f343f !important;
}