Node and npm\yarn
Install sass:
yarn global add sass
On linux, run build.sh
On win and linux, sass --no-source-map "./{THEME_FOLDER}/_main.scss" compiled/{THEME_NAME}.css", where {THEME_FOLDER} is folder of your theme and {THEME_NAME} is name of your theme.
Add --watch flag, if you want sass to recompile your theme on every change.
Install any extension that allow CSS injection, like this And throw your file at it.