No matter what changes you made to atom colors and so on, you can revert to the default values by choosing the 'Default atom properties' pop-up-menu command. It is present in the pop-up menu on bonds, atoms and in the clear space:
Click in some empty space in the drawing area with the left mouse button, and keep pressed down without moving.
Select the Space Popup → Default Atom Properties menu command
Your picture should now look as before.
