custom.css: make donation block larger on small screens

www/static/css/custom.css

@@ -141,6 +141,12 @@ padding: 0 15px;
   margin :auto ;
 }
 
+@media only screen and (max-width: 768px) {
+	#donation {
+		width: 80%;
+	}
+}
+
 #donation .btn em {
     font-style:normal;
 }