FadingMusic on "[Theme: Hueman] Post thumbnail proportions"

Hello! I'm using the Hueman theme in a very simplified setting for a small personal blog. The home page and all the category pages use the same layout.

http://bghandmade.com/category/albums/

The problem I'm having is this: on all the "listing" pages (home/category), the post thumbnail shows as a thin vertical image, instead of a landscape photo.

I found out (by playing around with "Developer tools") this can be influenced by setting background-size: contain; instead of cover to the .post-thumbnail. However, I'm not sure where to reflect this change in the actual CSS file, since that piece of styling is embedded (hard-coded?) in the HTML source.

Also, that change would also require to change the implicitly set height of the post thumbnail image, as well as the div.post-thumbnail, I suppose?

I'm not sure how that would influence the responsive design and should I edit somewhere else in the css file?

Thanks!

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