If you are using a dark background, or an image background for the body of your template you may find that your editor window is too dark. The same also applies to your print preview. In both cases, the background is being picked up from your template's CSS file.
To change this, add the following in your template's CSS:
In some templates you will see the following in your CSS file:
If you change it to this, you should also get a white background:
Experiment a little as each template uses CSS a little differently.