How to edit properties of whiskers, fliers, caps, etc. in Seaborn boxplot

EDIT: Note that this method appears to no longer work for matplotlib versions >=3.5. See the answer by @JohanC for an up to date answer

You need to edit the Line2D objects, which are stored in ax.lines.

Heres a script to create a boxplot (based on the example here), and then edit the lines and artists to the style in your question (i.e. no fill, all the lines and markers the same colours, etc.)

You can also fix the rectangle patches in the legend, but you need to use ax.get_legend().get_patches() for that.

I’ve also plotted the original boxplot on a second Axes, as a reference.

import matplotlib.pyplot as plt
import seaborn as sns
    
fig,(ax1,ax2) = plt.subplots(2)

sns.set_style("whitegrid")
tips = sns.load_dataset("tips")

sns.boxplot(x="day", y="total_bill", hue="smoker", data=tips, palette="Set1", ax=ax1)
sns.boxplot(x="day", y="total_bill", hue="smoker", data=tips, palette="Set1", ax=ax2)

for i,artist in enumerate(ax2.artists):
    # Set the linecolor on the artist to the facecolor, and set the facecolor to None
    col = artist.get_facecolor()
    artist.set_edgecolor(col)
    artist.set_facecolor('None')

    # Each box has 6 associated Line2D objects (to make the whiskers, fliers, etc.)
    # Loop over them here, and use the same colour as above
    for j in range(i*6,i*6+6):
        line = ax2.lines[j]
        line.set_color(col)
        line.set_mfc(col)
        line.set_mec(col)

# Also fix the legend
for legpatch in ax2.get_legend().get_patches():
    col = legpatch.get_facecolor()
    legpatch.set_edgecolor(col)
    legpatch.set_facecolor('None')

plt.show()