If you right click on the graph and select Advanced, Customize.... you can edit the appearance of controls.
To remove the rounded edges click on the little spanner icon near the top of the screen, it will change to a pair of tweezers.
If you then shift-right click on the background of the graph you can change its colour or make it transparent.
When you're done just 'apply changes' from the File menu.
I've played around with pasting in alternative pictures to get a customised control but have found this can slow down my code a bit. I've had more success using the plotimages.back property node to make graphs look pretty.
Hope that helps,
Ian