SASS & Themes and local vs Github


I am completing the tutorial on Jekyll. I have run into trouble with the CSS/SASS.

When running it locally using bundle exec jekyll serve I get an error message saying it cannot find it.

When I run it on GitHub I think it defaults to a theme listed the _config.yml. I have two questions - does the theme override the CSS call in _layouts\default.html? How do I get the page to find the css/sass file. Github repository here ->
Any help would be great.