Don't screw with tab height on non-Australis themes, provided that the stars align correctly during the startup process.

common/content/dactyl.js

@@ -1894,6 +1894,12 @@ var Dactyl = Module("dactyl", XPCOM(Ci.nsISupportsWeakReference, ModuleBase), {
             });
         }
 
+        if (config.has("default-theme") && "CustomizableUI" in window)
+            overlays.overlayWindow(window, {
+                append: [
+                    ["window", { id: document.documentElement.id, "dactyl-australis": "true", xmlns: "xul" }]]
+            });
+
         dactyl.timeout(function () {
             try {
                 var args = config.prefs.get("commandline-args")

common/modules/config.jsm

@@ -64,6 +64,12 @@ var ConfigBase = Class("ConfigBase", {
                            true);
         });
 
+        // FIXME: May not be ready before first window opens.
+        AddonManager.getAddonByID("{972ce4c6-7e08-4474-a285-3208198ce6fd}", a => {
+            if (!a.isActive)
+                config.features.delete("default-theme");
+        });
+
         services["dactyl:"].pages["dtd"] = () => [null, cache.get("config.dtd")];
 
         update(services["dactyl:"].providers, {
@@ -536,7 +542,7 @@ var ConfigBase = Class("ConfigBase", {
      *    dactyl.has(feature) to check for a feature's presence
      *    in this array.
      */
-    features: RealSet(),
+    features: RealSet(["default-theme"]),
 
     /**
      * @property {string} The file extension used for command script files.

common/skin/dactyl.css

@@ -227,12 +227,13 @@ statusbarpanel {
 @-moz-document url(chrome://browser/content/browser.xul) {
 
 /* Fix ginormous Australis tabs. */
-.tab-background > * {
+[dactyl-australis=true] xul|tab.tabbrowser-tab .tab-background > * {
     min-height: 24px !important;
     max-height: 24px !important;
 }
 
-.tab-background > :-moz-any(.tab-background-start, .tab-background-end)::after {
+[dactyl-australis=true] xul|tab.tabbrowser-tab
+    .tab-background > :-moz-any(.tab-background-start, .tab-background-end)::after {
     background-size: 30px 24px !important;
     max-height: 24px !important;
     min-height: 24px !important;