This is a work-in-progress. It's pretty basic and doesn't cover everything, but should help you avoid basic syntax errors. If anyone wants to contribute to this repo, by all means go ahead and completely rewrite it if necessary.
To install, work out where your .vscode extensions directory is and copy the whole repo (including the directory) into there.
On Windows, the directory should look something like this:
c:/Users/yourname/.vscode/extensions/
On other operating systems you'll need to look up where the vscode extensions is, using a search engine. Then let me know how you got it all to work (by opening an issue in this repo), and I'll include your instructions in these instructions. :)
After copying this repo into the extensions directory, close and restart VSCode.
It should now be syntax highlighting ".acss" files.
If you do not use VSCode and wish to get syntax highlighting ported over to your text editor, please raise an issue in this repo.