@pitaj that's it! Oh gosh, I literally couldn't see the connection of that ID. But now it all makes sense.
@paladin884 what do you mean that won't work? What did you try?
Here's me importing a CSS fine directly into less in one of my plugins:
Then you just need to make sure your less is defined in plugin.json:
You can instead just add it to your plugin.json under css like so:
Either of those options will result in it being added to the main css file loaded in header.tpl
Yes! That's it, didn't realised grunt was setup in a way to pick-up custom themes. It's working like a charm!
Thanks a lot @PitaJ !