The files in /core/plugins/twig/tpl . control the output of the .html files found in the website folder public_html. You should never modify these files, nor any files in core. Instead to override the theming you should copy and modify that directory, e.g.
cp -R core/plugins/twig/tpl .
For css changes you should edit /tpl/style.css in the newly created /tpl folder. Or the SASS files as desired.