Hi bernadette 
Some pages have specific classes that allow targeting with CSS, some don’t, and there may be javascript solutions to add classes to certain pages which a specific class.
I’d be happy to help you, but I need more information:
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Can you share screenshots that show your issue?
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What do you mean by “a single page of the category”?
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Can you provide a link (feel free to replace the domain name if you don’t want to share your forum URL) that would lead to this page?