Fix bugs with dark mode panels

[larsoner]

#14108 unsurfaced that we don't consistently or correctly honor dark mode in our subpanels. This PR fixes it:

from pathlib import Path
import matplotlib.pyplot as plt
import mne

mne.viz.set_browser_backend("matplotlib")
fname = Path(mne.__file__).parent / "io" / "tests" / "data" / "test_raw.fif"
raw = mne.io.read_raw_fif(fname, verbose="error").crop(0, 5).pick("mag").load_data()
raw.set_annotations(mne.Annotations([1.0], [0.5], ["bad_x"]))

fig = raw.plot(group_by="selection", theme="dark", show=False)
for key in "?ja":  # help, SSP projectors, annotations
    fig._fake_keypress(key)

In this PR gives

Don't think a test is needed for this one but could add if needed. Drafted changes with Opus 4.8.