How can I make the distinction between category and sub-item more clear?

I feel that the sub-items and categories are not distinct enough, so when I test my design I feel it's sometimes unclear which is a category and which is a sub-item, until I force myself to notice the subtle differences.

I want to make it immediately obvious that sub-items are subordinate to categories

I've tried using a darker background color for the sub-items to signify that they are inside the categories, but it seemed to push them outside the categories rather than showing that they are within them.

I've also tried indenting the sub-items, but it sacrificed vital space and looked unpleasant.

Is there any standard solution to this problem? Or is it just sufficient that the category and sub-item labels are different?