How do I modify hugo's built-in shortcodes?

Hello, I’m brand new to Hugo. I’m trying to have edit Hugo’s instagram shortcode to make it smaller. Ideally I want to follow this forum’s advice: [OT] CSS: How to make multiple Instagram pics in one row? problem is what do they mean when they say edit css? I don’t have that path on my folder. Best I have is under /public/css/ but that folder is complete empty. I tried adding a new css file to it and pasted the css code suggested in that forum but it’s not working correctly. any advice?

In Hugo that part is up to the theme you are using. See the themes documentation for how to customise its css.

Thank you very much. That helped a lot. I was able to dig in my theme’s files and looked at where everything is!