I believe you stumbled upon a BUG.
@weaver, The per post style box inserts the wrong post ID for a parallax post.
if you put a rule like
.parallax-content {color:yellow;}
In the per post style box of a parallax post with the ID 8200 for example, it will generate a rule
#post-8200 .parallax-content {color:yellow;}
Unfortunately, it appears parallax post have their ID modified to include -p in this case ID=post-8200-p
As a result, the generated rule has no effect