NB I have found a workaround for this, see below

Summary

At the moment

import numpy as np
import matplotlib.pyplot as plt

plt.style.use('dark_background')

X = np.linspace(0, 10*np.pi, 1000)
Y = np.sin(X)

fig, ax = plt.subplots()
ax.plot(X, Y)

plt.show()

displays

Proposed fix

It would be cool if the toolbar blended in with the graph!

Workaround

[UPDATE]

Turns out it's the same issue as #31177 (in the linked issue you will find a workaround which also worked for this issue in my case)