Print available rtemis themes
Value
Called for its side effect of printing available themes.
Examples
available_themes()
#> Available themes:
#> "white", "whitegrid", "whiteigrid,
#> "black", "blackgrid", "blackigrid",
#> "darkgray", "darkgraygrid", "darkgrayigrid",
#> "lightgraygrid", "mediumgraygrid"
