I can't address your specific theme, but for my theme I did this to get rounded corners on my Homepage images and Collection Images:
In file stylesheet.css.liquid find : .box-ratio
Add: border-radius: 8px;
A good way to find the CSS that affects your images is to use the Firefox browser Web Developer > Inspector tool to click on an image and see the CSS that affects the image's container. Then you can search for that container's CSS in stylesheet.css.liquid and add the border-radius code.
Or just post your shop's URL and me, or someone else will quickly identify the CSS you need to modify.