Follow this clicking path:
Online Store (from sidebar admin) > Customize (at right) > drop-down for pages, select Product Pages (to right of theme name at top) >
Product Pages (left nav list) >
scroll down to Featured Image (under Tab Content w/ Select Page button) >
Zoom Type drop-down >
select Zoom-In (not Lightbox, which is similar to a larger version of image opening in the browser window, except it has a close button instead of the default option which doesn't have a close button).
If tried that already, or after trying it, and there's no Zoom-In or Lightbox option, I suggest a different theme or waiting for help to get a zoom function going. It may also be in the Shopify help section. Can you post if anything worked in the topic here?
If Solo has no image zoom and/or you want a better product image experience, you can check for image zoom apps the Shopify app store.
You can try our app free, Magic Zoom Plus, which has just been picked as a Featured app by the Shopify staff:
As well as image zoom, it can show product videos and 360 spins.
I don't found Solo theme : https://themes.shopify.com/themes?q=solo