In the Design section of the Customizer you will find several options that allow you to change the look and feel of the site aswell as to add functionalities to it.

Theme
Sidebar Positioning
Menu Type
Post Archive Layout
Additional Options

Theme

Change the site to a Light or Dark theme.

Light theme helps in bright environments for superior readability and focus, making them ideal for daytime use and long texts.

Dark theme reduce glare, save battery (especially on OLED screens), and decrease eye strain in low-light settings, improving sleep quality and comfort at night, but can hinder long-form reading for some users. 

No Sidebar to keep the site at Full-width and let users navigate the site using the main menu at the top.

Left or Right Sidebar places a sidebar on the left or right side throughout the whole site.

Post Archive Layout

This dropdown menu gives you three choices for the layout of Archive Pages:
Big images, Small images and 3 columns.

Big images

Small images

3 columns

Additional Options

Search icon
Show a search icon in the top menu when selected.

Show related posts
When checked it shows related posts below the content of a Post.

Fixed Header
If checked the menu bar will stick to the top of the page when scrolling down.