I was searching dark themes for Matplotlib and found these three. And put them here with examples so I can find them faster next time.
QB style

Website: github.com/quantumblacklabs/qbstyles
Install:
pip install qbstyles
Example code:
import matplotlib.pyplot as plt
from qbstyles import mpl_style
def plot(dark):
mpl_style(dark)
plt.plot([1, 3, 9, 5, 2, 1, 1], marker='o')
plt.plot([4, 5, 5, 7, 9, 8, 6], marker='o')
plt.show()
plot(dark=True)
ing-theme-matplotlib

Website: pypi.org/project/ing-theme-matplotlib/
Install:
pip install ing_theme_matplotlib
Example code:
import matplotlib.pyplot as plt
from ing_theme_matplotlib import mpl_style
def plot(dark):
mpl_style(dark)
plt.plot([1, 3, 9, 5, 2, 1, 1], marker='o')
plt.plot([4, 5, 5, 7, 9, 8, 6], marker='o')
plt.show()
plot(dark=True)
mplcyberpunk

Website: github.com/dhaitz/mplcyberpunk
Install:
pip install mplcyberpunk
Example code:
import matplotlib.pyplot as plt
import mplcyberpunk
def plot():
plt.style.use("cyberpunk")
plt.plot([1, 3, 9, 5, 2, 1, 1], marker='o')
plt.plot([4, 5, 5, 7, 9, 8, 6], marker='o')
mplcyberpunk.add_glow_effects()
plt.show()
plot()


def plot_lines_glow():
plt.style.use("cyberpunk")
plt.plot([1, 3, 9, 5, 2, 1, 1], marker='o')
plt.plot([4, 5, 5, 7, 9, 8, 6], marker='o')
mplcyberpunk.make_lines_glow()
plt.show()
def plot_underglow():
plt.style.use("cyberpunk")
plt.plot([1, 3, 9, 5, 2, 1, 1], marker='o')
plt.plot([4, 5, 5, 7, 9, 8, 6], marker='o')
mplcyberpunk.add_underglow()
plt.show()