How can I alter the site styles without the change affecting every page?

I'm a beginner and trying to stylize my pages individually but seems like squarespace wont let me do this. What am I missing?

Can you describe in detail what you want & share page url? We can help easier

Stylizing individual pages, that usually requires code specific to the page either using a code block or advanced header code injection, and that requires a business billing plan. I suspect you are not missing anything using standard configuration options.