Update Ya.css for standardization of the icon sizes (#3144)

The added code standardizes all icon sizes to 16px, relieving extension makers from concerns about their extension's icon size.

appinventor/appengine/war/static/css/Ya.css

@@ -44,6 +44,12 @@ body, html {
   min-height: 100vh;
 }
 
+/* Standardization of icon sizes to 16px */	
+.gwt-Tree img, .ode-Icon img, .ode-Icon-selected img {	
+  width: 16px;	
+  height: 16px;	
+}
+
 /* Needed for Safari to position comments correctly */
 body.blocklyMinimalBody {
   min-width: auto;