Yes, "a" stylesheet - can be the main one or an individual mod's stylesheet.

To get the additional images displayed just below the main image instead of at the bottom of the page, you first need to move the additional images block of code up in
/includes/templates/your_template/templates/tpl_product_info_display.php.
Then you can style #productAdditionalImages {} and .additionalImages {} as desired for fine-tuning.