File path: skin/design/frontend/MageCheck/Tutorial/layout/local.xml
File path: app/code/MageCheck/Tutorial/view/frontend/magecheck_tutorial_index_index.xml
File path: skin/design/frontend/MageCheck/Tutorial/css/custom.css
File path: app/code/MageCheck/Tutorial/view/frontend/web/custom.css
The last thing you have to do is to clean and flush your magento cache:
php bin/magento cache:clean
php bin/magento cache:flush