Add brand colors to metainfo for Flathub

data/cz.bugsy.roster.metainfo.xml.in

@@ -25,6 +25,11 @@
   <!-- Use the OARS website (https://hughsie.github.io/oars/generate.html) to generate these and make sure to use oars-1.1 -->
   <content_rating type="oars-1.1" />
 
+  <branding>
+    <color type="primary" scheme_preference="light">#8b9dbc</color>
+    <color type="primary" scheme_preference="dark">#173c7a</color>
+  </branding>
+
   <screenshots>
     <screenshot type="default">
       <image>https://git.bugsy.cz/beval/roster/raw/branch/master/screenshots/main.png</image>