On Magento 2.3
After changing the theme, you need to create all the widgets.
If you are lazy as I am to recreate all the content in the backend, you can easily change the value in the database.
1) Every theme has a theme_id which could be easily found in the database table theme
2) Once you got the theme_id, open the widget_instance table and update the ...
The file is being inherited from the parent theme. If you bring into your theme after you static-content-deploy your version will be used instead.
Depending on what mode magento is in you might need to delete the file in view_preprocessed.
If in doubt search in /vendor folder for styles-l.css
Should be inside a magento blank theme folder.