Mr.Dev. is your provider of developing tools!
He gives you a boilerplate theme for Wordpress Blocks Full Site Editor and optionally for Mr.Dev's Framework.
Because it's a boilerplate, Mr.Dev.'s Theme does not come with default block templates (you can use the FSE to create templates for 404, archive, search, etc), it also removes the default colors from the full site editor and custom colors from the block editor (you can use the FSE to add custom colors).
It also has the bare minimum CSS styling needed to be compatible with default styles from Blocks, Mr.Dev and variables from Mr.Utils.
By adding Mr.Dev.'s Framework, you will be able to use the plugin interface to add theme colors, typefaces, sizes, stylesheets, javascripts and much more.
As oppose to Mr.Dev_s, Mr.Dev.'s Theme does not add styling or layout options to the framework either. If you want some default styles, you can enable them on the framework instead.