Thank you very much, Michael. However that somewhow did not work for me. After lots of experimenting and reading, I found the below to work (the repo referred to in the Jekyll documentation helped me in this):
Any partial scss file needs to reside in Jekyll’s sass library, which defaults to /_sass. In this library the scss partial with the leading underscore is processed, and imported into the using scss files via @import "FILENAME"; (here the filename w/o leading underscore).