If you've used other Shopify themes prior to setting up your storefront with Flex, then you may be familiar with "theme styles" (previously known as presets). In other Shopify themes, theme styles are different looks that you can choose from within a single theme. Each theme style will have a default set of fonts and colors, which can help you set up your shop.
In the Flex theme, we currently have 12+ demo shops as theme styles. We've taken the theme style concept a step further by creating the demo shop import feature in our Theme Updater App that allows you to fast-track a shop setup based on a demo shop of your choosing.
This means that all of the theme settings, layouts, content, and sections used in the demo shop of your choosing will then get applied to your theme. You can even add in the placeholder images used in each Flex demo shop in your own shop.
Changing Theme Styles
Since the concept of "theme styles" in the Flex theme differs from other Shopify themes, there is no "change theme style" button with the theme editor.
Instead, if you're looking to change your theme to use a different demo shop/theme style, there are two options.
Use the demo shop import feature
The demo shop import feature (via the Theme Updater App) will allow you to quickly change your theme to use the colors, settings, sections, and content of one of the Flex demo shops. Please note that importing a demo shop will overwrite any existing theme settings, sections and options. You may want to create a duplicate of your theme first if you'd like a backup.
Upload an alternate theme zip file from the 'unzip-me-first.zip' file
When you purchased the Flex theme, the theme downloads as a zip file called 'unzip-me-first.zip'. Once the file is unzipped, a folder called Welcome to Flex gets created.
There will be a main flex.zip file, as well as zip files named after the current demo shops (ex: flex-swagger.zip): 
If you want to start off with a different demo shop as your theme style, you can upload your demo shop zip of choice to Shopify. The theme will install with all of the sections, content, and settings as seen in that demo shop.
Please note - this will install a brand new theme to your Theme Library panel.