There are many themes that you can use for a boutique site, but you need an ecommerce plugin to sell online on WordPress. We don’t yet have a specific boutique theme available for download, but if you check out our WordPress Ecwid store setup, that explains the process for an ecommerce site set up on WordPress. Ecwid is a very simple ecommerce plugin for WordPress that has a ton of order management features built in. And it works with a huge number of themes, including many free themes plus paid themes like Divi and those from MyThemeShop. Most are mobile compatible these days.
Sorry that I don’t have a step-by-step with a specific downloadable theme, but once you jump in, it’s not hard to figure out. Most have excellent tutorials that lead you through the process. Good luck!