In the Debut theme, there should be a theme.scss.liquid file in the assets folder and that is the default one. If you don't have or accidentally deleted then download the debut theme and create theme.scss.liquid file, copy and paste the code. That will solve your issue.
Most likely this is the reason there is no theme.scss.liquid file: https://www.shopify.ca/partners/blog/deprecating-sass. This means there should only be a theme.css file. The same instructions for adding a font can most likely be used, with the exception of if the instructions use anything sass specific and not native to css.
What Dannytlake says, SASS has been deprecated from store files, so all files containing css will by default now only contain native css. Also, the liquid code has been moved to another file so the theme.scss.liquid's content now seems to have been moved to the two following files: