Fix #701 Preferable default theme (#746)

* Make it possible to update users.preferred_theme to NULL

* Make preferred them null when 'Default theme' is selected

* Run cargo fmt

src/routes/user.rs

@@ -399,7 +399,10 @@ pub fn update(
         )
         .0,
     );
-    user.preferred_theme = form.theme.clone();
+    user.preferred_theme = form
+        .theme
+        .clone()
+        .and_then(|t| if &t == "" { None } else { Some(t) });
     user.hide_custom_css = form.hide_custom_css;
     let _: User = user.save_changes(&*conn).map_err(Error::from)?;