Allow the user to set ggplot2 theme and colors

[gl-eb]

A ggplot2 theme object could be passed as an argument, or we could check whether a theme other than the default has been set in the current environment.

Palettes and colors would also be passed as arguments.

[gl-eb]

yardlistr::yardlistr() will now respect global ggplot2 theme set using ggplot2::theme_set()