Bath
A small, wet, brightly lit room — three things the aesthetic has to work around.

The bathroom is the easiest room in the house to change completely and the hardest to keep looking good, because everything in it is fighting humidity. It is usually small, usually lit from directly overhead, and usually rented, which sounds like three obstacles and is really three reasons it is the best room to start with: the surfaces are few, the pieces are cheap to swap, and almost nothing needs a screwdriver.
What to look at
Black fabric shower curtain
The largest single surface in most bathrooms, which makes it the one purchase that changes the room. Fabric needs a liner behind it.
Shower curtain liner
Cut slightly narrower than the curtain so it tucks inside the tub. Weighted hems stop the clinging.

Rustproof shower curtain hooks
Twelve to match the standard grommet count. Rustproof matters more here than finish does.

Black towel set
Ring-spun cotton. Read the care note before you buy: dark towels and benzoyl peroxide do not coexist.
Coffin bath mat
The shaped mats are the goth-native option. Check for a non-slip backing rather than assuming one.
Bat bath mat
Same category, softer read than a coffin. Machine-washable backing is the thing to verify.
Apothecary storage jars
Lidded glass for cotton, salts and swabs. Glass wipes clean and does not mind the humidity.
Dark bathroom accessory set
Soap dispenser, tumbler, dish. Resin sets are common here and are fine on a counter, unlike in a kitchen.
Arched window mirror
The cathedral-arch shape does more gothic work than almost any other single object.
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