My red"isch" variant of the slate theme.

Install it in the Frontend section of the Home Asssistant Community Store.

Copy the themes/slate_red.yaml file to a local folder called themes on the same folder level as your configuration.yaml.

Also include the following in your configuration.yaml:

frontend:
  themes: !include_dir_merge_named themes

Restart Home Assistant and select your theme by clicking on your user's profile circle in the bottom left.