You simply go to
jqueryui.com/themeroller/ and either download an existing theme, or create a custom theme.
After that, you upload the new theme (unzipped) to /components/com_virtuemart/themes/Dynatabs/templates/product_details/includes/js/css/
You will see that there are two folders there. Add yours.
Then in components/com_virtuemart/themes/Dynatabs/templates/product_details/
find the correct image handling option such as
dynatabsDark_multiZoom.tpl.php
In the tpl.php file, find this line:
<link rel="stylesheet" href="<?php echo VM_THEMEURL; ?>templates/product_details/includes/js/css/dark-hive/jquery-ui-1.8.5.custom.css" />
and change it to the custom file path that you uploaded. If you are doing a "dark" theme, you should choose one of the "dark" image handling options to make the CSS work with your theme. If you are using a "light" theme, then do the same.
That should do it. FTP is the easiest way to upload the files.