this stuff is missing from the theming

assets/private_browsing.VisualElementsManifest.xml

@@ -0,0 +1,12 @@
+<!-- This Source Code Form is subject to the terms of the Mozilla Public
+   - License, v. 2.0. If a copy of the MPL was not distributed with this file,
+   - You can obtain one at http://mozilla.org/MPL/2.0/. -->
+
+<Application xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance'>
+  <VisualElements
+      ShowNameOnSquare150x150Logo='on'
+      Square150x150Logo='browser\VisualElements\PrivateBrowsing_150.png'
+      Square70x70Logo='browser\VisualElements\PrivateBrowsing_70.png'
+      ForegroundText='light'
+      BackgroundColor='#14171a'/>
+</Application>

mk.py

@@ -100,7 +100,8 @@ def build(debug=False):
             #patch('../assets/disable-verify-mar.patch')
             patch('../assets/tryfix-reslink-fail.patch')
             patch('../assets/fix-l10n-package-cmd.patch')
-            
+            exec("cp -v ../assets/private_browsing.VisualElementsManifest.xml ../assets/PrivateBrowsing_150.png ../assets/PrivateBrowsing_70.png browser/branding/librewolf")
+
             # perform the build and package.
             exec('./mach build')
             exec('./mach package')