The best way is to right click anything and use “inspect element” which should help find CSS classes or IDs. You can then override easily with !important in your user stylesheet.
The stones and lines are drawn with HTML canvas so I’m not sure if it’s easy to swap images using CSS alone. But here’s a quick script that will hopefully help, it does nothing but replace the stones with gems … https://greasyfork.org/en/scripts/405885-ogs-custom-stones
To swap with some other images, just install with Tampermonkey, open the dashboard and open the script source code, then replace the two imgur URLs in the script with your own images, and save the script. It should work… but if it doesn’t I can try to help
(actually I thought this would be ugly but I think I kind of like it with the dark background 
)
Changing the color of the lines I think is as simple as this Javascript:
data.setDefault("custom.line", "pink");
(data is a global from OGS source I think so this can go anywhere)
edit: I think custom line color only works with the “plain” style so best if you are also overloading the background… probably a better way but too sleepy to read more OGS source for tonight
I’m not sure where the color for the circle on the last moved stone is but I think it’s also Javascript