BUG: Min/max markers on box plot are not visible with 'dark_background' theme
The min & max markers (the short lines at the ends of the whiskers) aren't visible with the 'dark_background' theme

#### Code Sample, a copy-pastable example

```python
import pandas as pd
import matplotlib.pyplot as plt

# Data is from the pandas plotting demo
# https://pandas.pydata.org/docs/getting_started/intro_tutorials/04_plotting.html
air_quality = pd.read_csv("air_quality_no2.csv", index_col=0, parse_dates=True)

plt.style.use('default')
air_quality.plot.box()

plt.style.use('dark_background')
air_quality.plot.box()

```

#### Problem description

The min & max markers (the short lines at the ends of the whiskers) aren't visible with the 'dark_background' theme

![112621690-f4f9ab00-8e21-11eb-9fcf-0c734f4d9831](https://user-images.githubusercontent.com/2547618/113479494-5c010a80-9487-11eb-87be-7e4b9596e9d6.png)

I initially reported this as matplotlib bug here: https://github.com/matplotlib/matplotlib/issues/19778

They concluded it was an issue with pandas.
diegopetrola said: "I also briefly investigated pandas and it seems they have a function called `_get_standard_colors()` in line 83 of the script called `boxplot.py`. I am fairly confident that it is this function that overrides the theme and causes the issue."

#### Output of ``pd.show_versions()``

<details>
INSTALLED VERSIONS
------------------
commit           : f2c8480af2f25efdbd803218b9d87980f416563e
python           : 3.8.6.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.4.72-microsoft-standard-WSL2
Version          : #1 SMP Wed Oct 28 23:40:43 UTC 2020
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : C.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.2.3
numpy            : 1.19.1
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 21.0.1
setuptools       : 49.6.0
Cython           : 0.29.21
pytest           : None
hypothesis       : None
sphinx           : 3.1.2
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : 1.0.1
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.13.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : 3.3.1
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pyxlsb           : None
s3fs             : None
scipy            : 1.5.2
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : None
</details>
