Bootstrap sub-theme override CSS
I'd wager that the stylesheet has not been added to the page. The documentation is pretty solid on this: Adding stylesheets (CSS) and JavaScript (JS) to a Drupal 8 theme.

This page isn't likely to go anywhere, but I'll summarise here. Please go and read the full documentation page though, there is MUCH more there:


  The general steps for loading assets (CSS/JS) are:
  
  
  Save the CSS or JS to a file. 
  Define a "library", which can contain
  both CSS and JS files. 
  "Attach" the library to a render array in a
  hook. 
  
  
  But in the case of themes, there is an alternative to step 3:
  themes can choose to load any number of asset libraries on all pages.
  
  Libraries
  
  [...] a library could just as well only contain CSS assets, or only contain JS assets. Most themes will probably want to have a global-styling asset library, for the stylesheets (CSS assets) that it wants to be loaded globally (i.e. on all pages where the theme is active — usually the whole site except the administration pages):

global-styling:
  version: 1.x
  css:
    theme:
      css/layout.css: {}
      css/style.css: {}
      css/colors.css: {}
      css/print.css: { media: print }

  
  Attaching a library to page(s)
  
  Attaching to all pages
  
  To attach a library to all the pages on the site that use your theme,
  declare it in your theme's *.info.yml file, under the libraries key:

name: Example
type: theme
core: 8.x
libraries:
  - fluffiness/cuddly-slider