Default colors should be replaced when +theme_blackboard() is added to ggplot object

[rishabhshah-92]

A suggestion: the default color for lines, points, etc. should be changed to white as part of the theme_blackboard, theme_abyss, etc. themes that have a dark background.

[bwiernik]

Really wish this PR would get merged into ggplot2... tidyverse/ggplot2#2749

[IndrajeetPatil]

Check out {thematic} package: https://rstudio.github.io/thematic/