jQuery: submit form after a value is selected from dropdown

$('#themes').change(function(){
    $('form').submit();
});

The other solutions will submit all forms on the page, if there should be any. Better would be:

$(function() {
    $('#themes').change(function() {
        this.form.submit();
    });
});

I recommend using the longhand bind method because it has the same effect as the shorthand supplied by the other answers, but you can add additional events if need be without having to change your code.

$("#themes").bind("change", function() {
  $("form").trigger("submit");
});

In case your html contains more than one form

$(function() {
  $('#themes').on('change', function(e) {
    $(this).closest('form')
           .trigger('submit')
  })
})