What I have seen for the most part is hiding the blizzard frame and replacing it with your own with it's own unique name. This would include functionality and not just visuals. This way other addons accessing the same frame won't affect yours.
However, if you just want to change the visuals and not the functional you might be able to do that by looking at the layout of the frame(s) in question and replace the art with your own.
Depending on what you were hoping to replace and with what would depend on what you should look into.
One step I would take is to see if someone has created a similar addon and see how they did it and learn from that.
Also, we have the blizzard code source here :
https://www.townlong-yak.com/framexml/live which is a good go to place to see how they do some of their frames ( that are available that is )