How to make theme based changes
Step 1: From the navigation bar on the left select Storefront Design and select Theme from the list of options
Step 2: Make the necessary changes. Fields marked with * are mandatory. These changes mainly affect the size of the images in a brand store.
Theme Directory: This field is to only enable older themes to be compatible with the new theme system. You can set the theme directory to use on the image size settings defined here.
Status: Set it to enable/disable based on preference
Lazyload image: This field is used to lazily load images on search and category page.
Default Items per Page: Determine show many catalogue items are shown per page.
List description limit: in the list view short description character limit
Add the required sizes for all fields under Images.
Step 4: Save the changes by clicking on the save button on the top right corner of the page.