consider this link
"Good mood" here, the word "good" clearly describes the word "mood". As far as I am concerned, I could change the word "good" in any other words that would be able to describe "mood".
On the latter part of the link is were my question comes.
"I am in good spirits" - We know that this particular sentence is correct. My question is, is "good spirits" considered as a compound word here? If not, why did it become correct, even if it lacks a determiner?
just to clarify
As we know, a link here explains that "in a good mood" was the correct usage, but it does not explain why we say "in a good mood" as oppose to "in good mood" just like in good company, in good standing, in good time, in good conscience, in good condition, in good stead which are all grammatically correct.
I really cannot find any suitable explanation for this thing.
By the way I already understand regarding the singular and plural matter.