Styling of the select tags for sorting topics is not possible without changing functions_content.php.
The evil seems to lie within the function gen_sort_selects(), where an array containing the HTML tags is returned. I would like to create a dropdown menu with a button, so the select tag would have to go, but for that I need to edit a file that I shouldn't have to edit for a template.
I think something like a loop would be much better for styling purposes. I would do it myself, but unfortunately my PHP knowledge isn't good enough.