In your Theme you should have several template files with names beginning with "content" followed by a hyphen, a unique name, and .php, e.g. "content-xxx.php". Sometimes they may be in a sub-folder called something like "template parts".
That's where you would find the content-product.php file and within it the loop that you can modify to add your custom style declaration.
However I would strongly encourage you to be certain you're modifying a CHILD Theme you've created, otherwise Theme updates will overwrite your edits.
Alternatively, you can add some code to your (Child) Theme's functions.php, or better yet a custom plugin that exists in your Plugins folder not your Theme folder (to make it Theme agnostic), to add classes.
You can do this using Filters, I've done it to add classes to the Body and Post class, but you can get more granular as you need to. See this page for some examples:
Scroll down to see the examples, you could target your template easily enough.