Ok the kilt is not a costume.

But mostly…
Do you think of it and use it as a garment?
Or, primarily as the foundation garment for an outfit – kilt with accessories?
A style. A fashion! (heh-heh).
Both? All?

I’m a newbie. I’ve been out and outfitted a few times, but mostly I wear the kilt at home.
Without accessories. I have them but they aren’t needed.
So I’m currently using the kilt mostly as a garment.

I regularly play/practice guitar at night. As the seasons change I change locations from my yard to my unheated garage and then in winter to my halfway-heated basement. (The rest of my family has the run of the house proper).

For the current cooler autumn temperatures in southeastern Pennsylvania I’ve switched from lightweight to medium-weight kilts (I don’t have a tank yet).

I’ve read that the first accessory is the sporran. Well yes for public and utility.
Although the accessories that I expect to eventually have the largest collection of are belts & buckles -- I just love all the choices.

But at home for me the first accessory is hose. Cotton, then wool.
And headgear. Cotton driving/golf caps, then leather, and finally a nice wool caubeen.
When I go to the watch cap then it'll be too cold for the kilt.

Maybe I’m putting together “at home” outfits after all.
I'll call them “sub-casual”.