CSS or Java Script code to Adjust Aspect Ratio for single Gallery Page

Hello. I'm using the Miller template and I'm not a seasoned coder by any means, yet I've made a concerted effort to solve this on my own with no success. 

I'm seeking guidance on how to create css or java script code that will allow me to override the aspect-ratio handle for a specific gallery page that's featuring one image.  The image is 16:9, yet the Miller template is using code that's forcing the image to be cropped to a square from edge to edge when viewed.  I don't want to use the global aspect-ratio handle to resolve this as the other gallery pages are perfect as they are.

I've tried using the code inspector tools with both firefox and safari to figure out what the handle is for aspect-ratio and have experimented with using different handles with no success.  With success using css, I can adjust the padding and the width as well as max-width, yet what I really need control of is the aspect-ratio handle as the other handles listed are not yielding desirable results.

Is there a handle for aspect-ratio that the user can manipulate via css or java script?  I've tried: {aspect-ratio: 50;} with no success.

Any guidance you have is welcome here.  Thank you kindly in advance for your help with this.