extensions-app: Add brand colors

Flathub recently added support for "brand colors"[0] that app
stores can use to improve an app's presentation.

Sam Hewitt was kind enough to provide some for the Extensions
app, add them to the metainfo.

[0] https://docs.flathub.org/blog/introducing-app-brand-colors/

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3226>

subprojects/extensions-app/data/metainfo/org.gnome.Extensions.metainfo.xml.in

@@ -30,6 +30,11 @@
     <kudo>ModernToolkit</kudo>
   </kudos>
 
+  <branding>
+    <color type="primary" scheme_preference="light">#c5eeb2</color>
+    <color type="primary" scheme_preference="dark">#447637</color>
+  </branding>
+
   <requires>
     <display_length compare="ge">360</display_length>
   </requires>