Drop the dark-purple ticks, legend keys, etc.

[stijnvanhoey]

I saw this in an email-communication of @florisvdh and as I agree with him an issue seems appropriate. These dark-purple ticks and other graph elements makes the plots more colorfull/crowded than necessary.

I know des goûts et des couleurs ..., so feel free to ignore the comment when there are good reasons to do otherwise.

On theming in general, I was impressed by the style package provided by BBC https://github.com/bbc/bbplot

[ThierryO]

The theme was designed by the in-house graphics designer.