Thunderbird のソースコード中の folderPane.css にあるフォルダーペインの色付けやフォントスタイル変更関連の記載は以下の通りになってます。
コード:
treechildren::-moz-tree-cell-text(hasUnreadMessages-true),
treechildren::-moz-tree-cell-text(folderNameCol, isServer-true),
treechildren::-moz-tree-cell-text(closed, subfoldersHaveUnreadMessages-true),
treechildren::-moz-tree-cell-text(folderNameCol, newMessages-true),
treechildren::-moz-tree-cell-text(folderNameCol, specialFolder-Inbox, newMessages-true) {
font-weight: bold !important;
}
treechildren::-moz-tree-image(folderNameCol, newMessages-true),
treechildren::-moz-tree-image(folderNameCol, isServer-true, biffState-NewMail),
treechildren::-moz-tree-cell-text(folderNameCol, isServer-true, biffState-NewMail),
treechildren::-moz-tree-cell-text(folderNameCol, newMessages-true),
treechildren::-moz-tree-cell-text(folderNameCol, specialFolder-Inbox, newMessages-true) {
color: var(--new-folder-color) !important;
fill: currentColor;
}
treechildren::-moz-tree-image(folderNameCol, hasUnreadMessages-true),
treechildren::-moz-tree-image(folderNameCol, closed, subfoldersHaveUnreadMessages-true) {
opacity: 1 !important;
color: var(--unread-folder-color);
fill: currentColor;
}
treechildren::-moz-tree-image(folderNameCol, newMessages-true),
treechildren::-moz-tree-image(folderNameCol, isServer-true, biffState-NewMail) {
opacity: 1 !important;
fill: -moz-hyperlinktext !important;
}
treechildren::-moz-tree-cell-text(folderNameCol, isServer-true, biffState-NewMail, selected, focus),
treechildren::-moz-tree-cell-text(folderNameCol, newMessages-true, selected, focus),
treechildren::-moz-tree-cell-text(folderNameCol, specialFolder-Inbox, newMessages-true, selected, focus) {
color: var(--new-focused-folder-color) !important;
}
treechildren::-moz-tree-image(folderNameCol, selected, focus),
treechildren::-moz-tree-image(folderNameCol, newMessages-true, selected, focus),
treechildren::-moz-tree-image(folderNameCol, isServer-true, biffState-NewMail, selected, focus) {
opacity: 1 !important;
fill: var(--new-focused-folder-color) !important;
}
treechildren::-moz-tree-cell-text(folderNameCol, noSelect-true) {
color: var(--noselect-folder-color) !important;
font-style: italic;
}
1つ目が未読フォルダーをボールドにするもの、2つ目が新着フォルダーの色付け、3つ目が未読フォルダーの色付けになります。
color: orange !important; とかしてやれば、新着バッジと同じような色合いになりますね。