By default on Ella Theme, on product page (mobile), image slideshow is displayed like this:
I want to make it look like this:
selected image needs to be highlighted with black outline #1e1e1e
It would need customization to implement this feature, Unfortunately, there is no simple solution otherwise I will be happy to help you.
I would suggest hiring a Shopify expert to assist you OR feel free to contact me.
This is pro theme theme there must be an option to do this go to online store click on customize section select product page check it on left sidebar