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

private-browsing-theme/README.md

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+# Private browsing theme
+
+## What it does
+
+An example using the theme API to apply a different theme on private windows.
+
+## What it shows
+
+How to use the windowId argument of browser.theme.reset() and browser.theme.update().

private-browsing-theme/background.js

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+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);
+  }
+}

private-browsing-theme/manifest.json

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+{
+  "manifest_version": 2,
+  "name": "Private browsing theme",
+  "version": "2.0",
+  "description": "Extension that sets a dark theme for private windows",
+  "background": {
+    "scripts": ["background.js"]
+  },
+  "permissions": ["theme"],
+  "applications": {
+    "gecko": {
+      "id": "private-window-theme@mozilla.org",
+      "strict_min_version": "57.0b14"
+    }
+  }
+}