You mean the color around your "real" content?

You can adjust that.

Just open your Website Maker, click "more" next to the disk-icon and choose the last entry in the box....should be something like "Options".

After that, you have the chance to adjust the colors. You can choose between "website-background" (which isn't necessary to you, I guess) and "browser-background", which should be the grey color you complain about.

But: You have to choose one color.