How to use theme settings for store design and functioning (cart, newsletter, and social media)?

Go to Online Store from your Shopify admin, and then click Themes.

After clicking on Themes, a new window will appear. Here, find the theme in which you want to add and customize sections, and then click Customize.

After clicking on Customize button, a Side Bar Window will appear at the left side of your screen.

Scroll down the side bar menu and click on Theme settings.