Thumbnails appear ugly and distorted after theme migration
It may happen that all of the new images that are uploaded after activating the new theme look good, but the old thumbnails appear ugly and distorted.
When you upload an image to WordPress, it creates duplicate images of the original in different sizes and saves them in the uploads folder. WordPress gets the thumbnail image size information from your theme. Your theme defines the thumbnail sizes, which are really useful in order to load the best image size when needed. These new image sizes are only applied to the images that you upload after activating the new theme.
To solve this problem you’ll need to generate new sizes for all the previously uploaded images. This process is often referred to regenerating thumbnails and there is a plugin to help: Force Regenerate Thumbnails.