fix plotstyle.add() to store updated plotstyle in the correct environment for plotstyle() to use it

[0UmfHxcvx5J7JoaOhFSs5mncnisTJJ6q]

Before

plotstyle.add('foo', 'bar', 'red') |>
    invisible()

plotstyle('foo', verbosity = 1)
# Brewed colors for 1 unknown entities:
# foo
#       foo 
# "#e6194B"

After

plotstyle.add('foo', 'bar', 'red') |>
    invisible()

plotstyle('foo', verbosity = 1)
#   foo 
# "red"