Revert to default view options

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.