[enh] add visually hidden but accessible css class - #350

searx/static/themes/oscar/less/logicodev/search.less

@@ -77,4 +77,14 @@ Ny0yNFQxMToxNTowMCswMjowMP7RDgQAAAAZdEVYdFNvZnR3YXJlAHd3dy5pbmtzY2FwZS5vcmeb
 
 .search-margin {
     margin-bottom: 0.6em;
-}
+}
+
+.visually-hidden {
+    position: absolute !important;
+    height: 1px;
+    width: 1px;
+    overflow: hidden;
+    clip: rect(1px 1px 1px 1px); /* IE6, IE7 */
+    clip: rect(1px, 1px, 1px, 1px);
+    white-space: nowrap; /* added line */
+}