What does "even in costume" mean here?

I was reading The Invisible Man, here's the sentence:

The stranger did not go to church, and indeed made no difference between Sunday and the irreligious days, even in costume.

What does even in costume mean here? I don't quite get it, since both Sunday and irreligious days are days, unlike costume which is some sort of cloth.

Original Chapter


Solution 1:

It means even in the way he dressed. It implies he didn't dress up on Sundays, like his churchgoing counterparts did.