[fix] several issues in the simple theme

- using more rem in style and definitions
- mobile width in preferences.less fix max-width: 75em to 80em (normalized with
  style.less and other)
- do not display #backToTop position on tablet (when max-width: 80em)
- fix answer box on mobile (when max-width: 50em)

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

@@ -122,7 +122,7 @@
   }
 }
 
-@media screen and (max-width: 75em) {
+@media screen and (max-width: 80em) {
   .preferences_back {
     clear: both;
   }