I have showered in the dark a time or two, and it can be a much-needed break for the senses. I also love to jump in the bath (pitch dark) with or without some soft music playing—and I'm doing that quite frequently lately instead of my usual showers as part of a self-calming routine. Many days, the very best I feel all day is when I'm in the shower, feeling the steaming hot water release tension in my back muscles. It's very relaxing for me. I spend a bit more time in the shower than an ordinary person might—just because it feels good and safe to me.
Anyhow, most people have something that helps them feel safe, and those safe things can be especially important to sufferers. I obviously don't know your partner, but my wild guess is that the shower—especially with the lights out—is a relaxing, safe place for him.
If your partner is in treatment and his "odd" behaviors are somewhat benign, I'd say don't mind those "odd" things . . . at least at this stage in his recovery. As he recovers he will naturally reduce his need for coping/calming routines.
I know it's really hard to understand the behaviors of a PTSD sufferer. PTSD affects not just many, but MOST aspects of our lives, in ways you truly wouldn't expect. We seem to make no logical sense—I know! There is a sense behind it, but the only way to fully understand PTSD to experience it yourself. Even my excellent therapist, skilled as he is, says I understand more about PTSD than he does. I'm so glad you don't have the disorder and it makes me feel good to read your posts on this site and see how much you care for your partner. Your search for knowledge and understanding is a beautiful expression of love.
Have you read any books out there written for those who love someone with PTSD? There's some great stuff out there, and I would be happy to offer suggestions, as I'm sure others here would too. :)
Peace and love.