Dark background theme

Thanks for plotnine, I'm a big fan!

I like working with a dark background. Using plotnine.themes.theme_dark gives this image, which has a lot of light area which doesn't work well on a dark background.

from plotnine import ggplot, qplot, aes, geom_point, geom_smooth, geom_bar, ggsave, geom_density, theme_set, theme_dark, theme_xkcd
import pandas as pd

theme_set(theme_dark())
df = pd.DataFrame({'a': [1,2,3], 'b': [4,5,6]})
ggplot(aes(x='a', y='b'), data=df) + geom_point()

Might it be possible to add a dark theme like theme_solarized_2 from ggthemes?

Start by locating plotnine.themes.theme_dark and reviewing how the existing theme is assembled. Compare the requested appearance with ggthemes' theme_solarized_2 reference; done means plotnine provides a dark theme that produces the requested low-light-background appearance with the example code.