You create the .info.yml file in the root of your theme folder. The folder should have the same name as the .info.yml file. So if your theme is named “Fluffiness” then the folder is named “fluffiness/” and the .info.yml file is named “fluffiness/fluffiness.info.yml“. If the file is present your theme will be visible in your website at Manage > Appearance (http://example.com/admin/appearance).
name: Fluffiness (Required)
type: theme (Required)
description: ‘A cuddly theme that offers extra fluffiness.’ (Required)
core: 8.x (Required)
sidebar_first: ‘Sidebar first’