[simple theme] define device width with less vars in defenition.less

searx/static/themes/simple/src/less/toolkit.less

@@ -433,7 +433,7 @@ select {
   }
 }
 
-@media screen and (max-width: 50em) {
+@media screen and (max-width: @phone) {
   .tabs > label {
     width: 100%;
   }