While the instructions for themeing MarketPress are clear and easy to follow, they don't work. Yes the custom CSS style will show up in store settings dropdown of styles but you can't choose it. From examining the marketpress.php I suspect style additions have to be made to the load store_theme function or it has to be set to an inclusive name variable somewhere. Then some change has to be made to a select statement to allow selection of styles other than icons.css.
Implying that the user can choose a custom style simply by making a custom CSS and putting it in the marketpress-styles directory is frustrating and misleading.
After some hour and a half work, I found a workaround that will let me choose my custom css but it is a nuisance if I want to switch back to the default icons.css file.
So either something is left out of the directions or we are not supposed to be able to use our custom styles without purchasing the pro version.