how to override theme settings/styles

I am currently making my website mobile compatible, but I am running into an issue where my theme is blocking my shop page from having 2 columns, having one instead. After asking the plugin developers they told me that the reason that I was having this problem was because of my plugin. Is there any way to edit this CSS manually through the custom CSS tab in the control panel? Any indication of where I may need to look would be great.
Good day

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