If the about page says Ultimate then that is what you are using - thats fine.

Under the themes tab if you highlight the Elegant Green theme, then select 'Open Folder' you will get the directory with the separator file in it.

You may be able to change this for your Black dot image (without seeing it I can't say), or alternatively make the default separator something unobtrusive e.g. a single pixel or transparent small image. Note - I've not tried this so can't say what would be the best approach.

To insert the separator I just right click the dock, choose Insert New Dock Item, then choose Separator from the submenu. That inserts the green line which I right click and choose Dock Entry Properties. Then I choose to Change Icon in the bottom right and pick my black dot icon.

https://ibb.co/mS33pYW
Ok, that didn't quite resolve the problem, but I might be able to figure it out from here. It elongated the image, but there might be a way to fix it. Let me know if you have any ideas.

Hm. It looks like the original separator is 12x40 pixels. Not sure how to change the size... Thanks for the huge help, I'm a lot closer. Any ideas on how to modify the size of the default separator?

You can either edit your black dot image to make it thinner, so that it looks better when it is stretched out in display.

Alternatively I just created a 1x1 pixel image and replaced the default separator with that - it effectively gave me a blank area in the dock.

You can either just keep it tlike that or then use your original method for inserting your black dot separator. You will need to accept the blank space being there though.

Ok then, I guess I'll just make a small black dot and it will have to do. Thanks again for all your help. I really appreciate it.

Sorry I had to go do some things - did you get something you are happy with?

I just put a quick black oval in as the separator to see what it looked like:
https://ibb.co/YfbPRvR

Anyway good luck

Yah, mine looked a lot like that. I couldn't get it to be round, so finally what I did was just to copy the sep.png over to my icons, and use it for all of the separators. I'd rather have a big black dot as a separator than the green line, but at least it's uniform now, and that's the big deal. So I'm happy, and I feel finished. Thanks for all your help.

Glad that that's solved - at least, roughly....