We'll open up the Customizer again via Appearance > Customizer. Now, it's best to click on an actual category so we can see the changes in real time. In this case, 'Music'.
Go to Layout > Blog Archive Settings and you'll see the specific options for this section.
In this clip, we're going to make a simple change and adjust the layout from a 'List' format to a 'Grid of Two'.
The other options available to you are as follows:
Default Archive Heading Image:
You can add a default image which will appear on each category. You can also add a unique image to a particular category by going to Posts > Category - clicking on one and adding an image via the 'Banner Image' option. The ones on the demo site are 1680 x 300 px.
Archive Heading Image Overlay:
Adds a dark (or any color background) behind the category title for better visibility. You can reduce the opacity down to zero if you'd prefer this not to display.
Show the Excerpt:
Shows a snippet of text from the article underneath the title
Default number of characters to display - defaults to 150.
Blog Archive Heading Background:
This will be revealed if you remove any archive heading image (the first option) - this is ideal if you want just a simple color for your header.
Changing the Sidebar:
These are widgets. They are accessible via Appearance > Widgets > Sidebar