turns out that we /do/ use tertiaryAccentColor after all

src/skins/vector/css/_common.scss

@@ -132,6 +132,10 @@ textarea {
     color: $secondary-accent-color;
 }
 
+#mx_theme_tertiaryAccentColor {
+    color: $roomsublist-label-bg-color;
+}
+
 .mx_Dialog_wrapper {
     position: fixed;
     z-index: 4000;

src/skins/vector/css/themes/_dark.scss

@@ -34,7 +34,11 @@ $secondary-accent-color: $primary-bg-color;
 // by overriding the key to something untintable
 // XXX: this is a bit of a hack.
 #mx_theme_secondaryAccentColor {
-    color: #2d2d2e ! important; // deliberately off by one
+    color: #c0ff33 ! important; // deliberately off by one
+}
+
+#mx_theme_tertiaryAccentColor {
+    color: #c0ffee ! important;
 }
 
 // used by RoomDirectory permissions