Improve label text rendering

[caarmen]

The css for the label is this:

.labelname {
  font-size: 0.7em;
  box-shadow: 1px 1px 2px #333;
  text-shadow: 1px 1px 1px #222;
  font-family: sans-serif;
  padding: 1px;
  color: #fff;
}

The text is white with a black text-shadow. The idea was to make sure the text could be readable on either a light or dark label color.

However, in reality, it's not very readable when the label color is light.

Maybe we can have two styles for the label text, one for light labels (with black label text) and one for dark labels (with a white label text -- or white text with the black text-shadow as we currently have).

[caarmen]

You can assign this one to me.