Ask How do you ensure a membership site has a consistent design across pages?

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I'm building a membership site and I want it to look professional. I'm using a template, but I'm worried about consistency. I see many sites with the same layout on every page. I'm trying to achieve this too. I've heard about using CSS and themes, but I'm not sure how they work. I'm looking for simple solutions. How do I ensure my membership site has a consistent design across all pages?