Consider changing the `brightblack` color and glyph of NULL bytes

I have a couple of issues with the display of the offset and NULL bytes. They are not such a big deal for me, because I do a manual build and I apply a patch to fix them, but I thought of bringing them up anyway, for the benefit of other users with similar terminal configurations.

• The first one, involves the color. brightblack is the background color of the Solarized Dark color scheme, so it becomes invisible on terminals with that configuration. This is a very popular color scheme, and I’m surprised that it hasn’t been brought up already. I would suggest to be changed to blue, which is a more compatible foreground color, and it’s not used for something else.

• The second, has to do with the choice of the display character for NULL bytes. The "diamond operator" (U+22C4) is not a very common symbol, and it's not present in some popular fonts, like Terminus and Adobe Source Code Pro, so a fallback font is used and it doesn’t look nice. I think a better choice would be a more common symbol, like "subscript zero" (U+2080).