Add per-window theme example: private browsing theme (#307)

private-browsing-theme/background.js

@@ -0,0 +1,25 @@
+browser.windows.onCreated.addListener(themeWindow);
+
+// Theme all currently open windows
+browser.windows.getAll().then(wins => wins.forEach(themeWindow));
+
+function themeWindow(window) {
+  // Check if the window is in private browsing
+  if (window.incognito) {
+    browser.theme.update(window.id, {
+      images: {
+        headerURL: "",
+      },
+      colors: {
+        accentcolor: "black",
+        textcolor: "white",
+        toolbar: "#333",
+        toolbar_text: "white"
+      }
+    });
+  }
+  // Reset to the default theme otherwise
+  else {
+    browser.theme.reset(window.id);
+  }
+}