I play around with the default theme sometimes, and I know there are discussions but here is just one colour to change which effects the hole filling. It's the Gird's colour. When working in orthographic view the gird is almost invisible. I changed it to 4E4E4E and it's so much better. Try it out yourself and tell me what you think.
(oh and I have the wire colour paler but this is not subject now ;))