The machine name must be all lowercase, start with a letter, use an underscore () instead of spaces, and contain no other symbols.

Examples of a good machine name are _icecream and _ice_cream.

We will choose icecream as example of theme name.

=> Create the file themes/icecream/icecream.info.yml

Here's what these key/value pairs do:

name (required) The human readable name of your theme, displayed in Drupal's UI when administrators are browsing the list of available themes

type (required) Tell Drupal what type of project this is. Required, and will always be set to 'theme' for a Theme.

description A short one-line description used in the UI when listing your theme.

package The package your theme belongs in; used for grouping projects together.

core (required) The version of Drupal core that your theme is compatible with. Required; for Drupal 8 themes this will likely always just be '8.x'.

base theme (default = Stable) The machine name of an installed theme to be used as a base theme.

If no base theme should be used, enter "false" as a value for this key.