Your Thinkific site can be customized using Site Builder.
Further customizations can be made using our Themes feature, but this is an option intended for use by developers familiar with HTML/CSS only.
Whilst we're able to provide basic guidance with using Themes, we're not able to provide support in terms of working on your theme directly, or providing coding instructions for making significant changes.
If you're looking to significantly customize your theme, we would recommend hiring a designer. We have some wonderful freelance web developers on our Thinkific Experts page who you can reach out to if you need! There's also the Thinkific Store on Fiverr, for smaller, quicker jobs.