Theme Pop-dark incorrectly themes text.

[motivator]

* I am running Pop!_OS 18.04 LTS.
* I installed Gnome Tweaks so I could change the theme for my GUI.
* I set the theme to `Pop-dark`.
* In several apps (ie. Firefox 61.0.1) the theme displays incorrectly. Text is often displayed as dark text on a dark background, so it is nearly impossible to read.
* The same problem exists for `Adwaita-dark` and `Pop-dark-slim`.

* Another related problem that shows up in `Text Editor` is that the current line a user is typing on is highlighted in a light color that is nearly indistinguishable from the text. This problem exists with all the same themes as the first problem.

[Jeremy Soller (jackpot51)]

This is a problem with the applications mentioned. Adwaita-dark is the dark variant of the default GTK+ theme, so if the problem exists there, the application must be addressed.