spurious text-only logo when default mode is "auto"

[drammock]

I think #1146 introduced a nasty side-effect:

here's the rendering (the text "logo image" is black-on-almost-black but it's there):

After switching to light mode, there's no logo at all (just the text). The relevant part of our project's conf.py is

html_logo = "_static/mne_logo_small.svg"

(i.e., we use a logo that works fine on both light/dark modes, so there is no separate logo specified per mode).

cc @lunar-debian @12rambau

[lunar-debian]

I’ll take a look, hopefully later today.