What are the minimum requirements for a custom Drupal 8 theme that can be enabled I'm testing creating new themes in Drupal 8 and I'd like to know the very minimum it takes to create one that can be enabled. I'm not worried about any actual styling yet.

I've created a theme folder in themes called uktheme and I currently have a uktheme.info.yml file in that folder.

the contents of that file are 

name: UK Theme
type: theme
description: 'Test theme'
package: Core

regions:
  header: Header
  primary_menu: 'Primary Menu'
  feature: 'Feature'
  content: 'Content'
  page_bottom: 'Page Bottom'
  footer_first: 'Footer first'
  footer_second: 'Footer second'
  footer_third: 'Footer third'


However I receive

The website encountered an unexpected error. Please try again later.


when attempting to go into the appearance settings. I do not believe it's my folder structure and location because I successfully created another theme copying the content of Bartik.
Perhaps you've missed core.
Required params in *.info.yml are:
name
description
type
core
regions:content  

Also you can check your *.info.yml for using tabs instead of spaces:


  Tabs are NOT allowed, use spaces ONLY.
  You MUST indent your properties
  and lists with 1 or more spaces.


See more in official docs: https://www.drupal.org/theme-guide/8/defining-a-theme