How do I attach CSS styles to all the pages?
Please try using below code
info.yml :

name: mysub-theme
type: theme
description: 'my sub-theme Commissions'
core: 8.x
base theme: businessplus_lite

libraries:
  - mysub-theme/my-sub-theme-style


mysub-theme.libraries.yml :

my-sub-theme-style:
  version: 1.x
  css:
    theme:
      css/style.css: {}