-Open your RO folder and locate 'data'
-then texture
-then effect
-locate the image named 'magic_target' and change it to your liking. I forgot the specific format but you can change it to almost anything I think?
Here is an example of what i changed mine too.
Edit: If you're using thoth.grf it should come with that cast circle automatically and thats something I dont know how to do. Probably have to open the grf and look around I guess.
Inochi's post in how to change spell circle was marked as the answer