The Debian package seems to be patched to make Audacity work properly with dark themes (thanks very much!). Oddly, the Tracks>Spectrograms panel of Preferences still does not work in this regard: the background comes up white with white text on my Adwaita-dark–themed GNOME desktop. To be clear, all the other Preferences panels come up as desired, with dark coloring.
Hello, I have the same issue, white text on white background, except more extensive; it's not only the Tracks>Spectrograms panel of Preferences, it's all the other Preferences panels as well. I have tried various dark gtk themes with no success. It apparently has been confirmed on Solus 3, Ubuntu 18.04, and Mint 19 as well (see https://forum.audacityteam.org/viewtopic.php?f=48&t=98471) so, upstream issue?