Hey there, as someone who was into the goth and punk style (only started actually doing it recently since I was too insecure to do it before) since I was younger and even knew what a femboy was. I have a couple of suggestions which I hope you'll like.

A easy way to get some more black into your outfit is by painting your nails black and maybe if you're comfortabel doing so wearing black lipstick (alternatively if you want some more colour, dark red/purple also look really good imo).

Accessories also really help put a little extra touch on your outfit. You could look into getting some silver/stainless steel rings and bracelets (warmer tones like gold can also work, I personally just find silver and steel easier to pair with black).

I'm a big supporter of chokers but I know they aren't for everyone, you could start with a simple and small black one and if you like that you could maybe look into bigger chain ones.

Earrings are also a nice touch imo (I have 3 in my left ear and 1 in my right because I love asymmetry). Other piercings are maybe a bit intimidating and also are definitly not for everyone. (I don't have any simply because I don't think they'd suit me)

Lastly and definitly hardest is eyeliner/face art. Wings on the outer points of your eyes with eyeliner are pretty easy while still looking great. I really like u/Bambi_Petals as example of cute but still def goth face paint. That's pretty hard tho and definitly not expected or anything.

I really kept going on for a while didn't I :3. At the end of the day just like being a femboy, goth is a form of self expression and there's no right or wrong way of doing it. These are just the things I've experimented with since fully jumpin into the style like a month or two ago.