replace plugin.png with scalable icon

css/defines.less

@@ -42,7 +42,7 @@ body.ttrss_prefs,
   font-family: 'Material Icons';
   font-weight: normal;
   font-style: normal;
-  font-size: 24px;  /* Preferred icon size */
+  font-size: 18px;  /* Preferred icon size */
   display: inline-block;
   line-height: 1;
   text-transform: none;
@@ -50,6 +50,7 @@ body.ttrss_prefs,
   word-wrap: normal;
   white-space: nowrap;
   direction: ltr;
+  vertical-align: middle;
 
   /* Support for all WebKit browsers. */
   -webkit-font-smoothing: antialiased;