Viridis is not good default colormap

Viridis has two drawbacks as the default colormap:

1. Words are not sequential, so there is no particular reason to use a sequential colormap.
2. The default background is black, and viridis (deliberately) gets very dark, and consequently hard to read. This unintentionally emphasizes random words.

The default colormap should be a qualitative colormap, that is, a colormap with equal perceptual weight, e.g. Dark2.