Read css variables from xaringan-themer.css for ggplot2 theme

[Garrick Aden-Buie]

Currently the ggplot2 theme feature uses env vars to retrieve key colors, which only works if the xaringanthemer style functions are called in the rendering session. Now that CSS variables are implemented, the ggplot2 theme should be able to read from xaringan-themer.css if the style functions are run separately or outside the slides Rmd.