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

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

@@ -196,6 +196,12 @@ html {
 @results-margin: 2rem;
 @search-width: 40rem;
 
+/// Device Size
+/// @desktop > @tablet
+@tablet: 80em;
+@phone: 50em;
+@small-phone: 35em;
+
 /// From style.less
 @stacked-bar-chart: rgb(0, 0, 0);