Got the same problem, and even with this fix it does not change , i still have a blank html with only my content but not the style. I was with antimatter before and want to switch my inheritance to quark. I tried to put directly quark and it’s working.
Here is my mytheme.php :
// class Mytheme extends Antimatter
class Mytheme extends Quark
// Some new methods, properties etc.
and my mytheme.yaml :
# - user/themes/antimatter
Any idea ? Thanks in advance !