Doc: How to set a theme

[ptoche]

An example of how to set a theme would be greatly appreciated. Something like this or better:

using AlgebraOfGraphics, CairoMakie
using Colors  # for the palette
set_aog_theme!()
update_theme!(
    Theme(
        palette = (color = ["#1f78b4", "#ff7f00", "#029e73"],) # DON'T FORGET THE COMMA HERE!
    )
)

df = (x=rand(["a", "b", "c"], 100), y=rand(100))
plt = data(df) * mapping(:x, :y, color = :x) * visual(BoxPlot)
draw(plt)