Add style for silver sponsor

Author: GuillaumeGomez

_sass/_layout.scss

@@ -512,6 +512,11 @@ ul.sponsors {
                 color: darken(#cb7401, 20%);
             }
 
+            .silver {
+                background: linear-gradient(0deg, #c4c4c4, #f7f7f7);
+                color: darken(#b37a4f, 20%);
+            }
+
             .bronze {
                 background: linear-gradient(0deg, #b37a4f, #f5caa7);
                 color: darken(#b37a4f, 20%);