I was just looking for something like this, so the topic was perfect timing… I like the flat style of GoKibitz, for some reason I find it calm and peaceful and friendly, so I wanted to replicate it in OGS. Specifically I really wanted the flat edges on the white stones, but since the OGS plain style has a black border on the white stones, and uses HTML canvas to draw the board, I had to make some JavaScript changes. In order for this to work, you will need to use Tampermonkey which injects a user script into the page. Here is the script.
I tested it with a quick bot game and a demo board, but I’m usually a backend programmer so there definitely might be bugs 
just a warning, if something looks wrong in the middle of a game, turn off Tampermonkey and refresh the page
edit: Oh and by the way I also found out you can do something like user scripts in iOS (I usually play on my iPad)… there is something called “Shortcuts” that can inject Javascript. It’s a lot harder to get it working but if anyone wants I can help