SpPaginatorPageMorph>>newContentMorph should use theme background

[terpon]

I started to fill an issue on pharo tracker, but I guess this should get here instead.

Working with dark themes, I was bothered by the inspector. When several pages are opened, small white blocks show the offscreen pages. I looked at UITheme and subclasses to see if I could change that, but that’s baked in the code.

The initialization method already accesses the theme for the border color. Instead of Color white the method should use the theme background color.