Modernize CSS build: replace Gulp with npm scripts, update to Dart Sass 3.0 (!220) · Merge requests · project / paragraphs

Note that this MR is based on !219 (merged) which needs to be merged first ino 8.x-1.x, then this MR rebased and then merged.

Modernize CSS build: replace Gulp with npm scripts, update to Dart Sass 3.0