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

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

@@ -103,7 +103,7 @@
   vertical-align: middle;
 }
 
-@media screen and (max-width: 80em) {
+@media screen and (max-width: @tablet) {
   #search {
     padding: 0 @results-tablet-offset;
   }
@@ -145,7 +145,7 @@
   }
 }
 
-@media screen and (max-width: 50em) {
+@media screen and (max-width: @phone) {
   #search {
     width: 100%;
     margin: 0;