Further exploration has meant that I have discovered that indeed previous issues are available under the archive tab, and that the widget does work on one of my other blogs that that doesn't have a black background. It appears that you can change the HTML coding to change the colour of the heading but that still didn't improve things for a black background.
I also changed the title of my newsletter after I produced it and it seems that the widget has embedded in it the original title and I can't work out how to change it.
However, I have explored the newsletter a bit more closely and realize that what I was looking at was really only my headlines! Now I realize that if I click for example on #blogjune the newsletter will give me links for many, presumably all for that day! Wow! this is a very useful tool. I have been reading #blogjune posts in a very scrappy way so this will give me any easy way to get to them and scan them.