For anyone else that has an issue reading text within the TLA editor due to a "dark" Gnome theme in Ubuntu, I found out that you can keep Gnome using the "dark" theme you have chosen and simply launch the Toolbox with the following command:
GTK_THEME=Adwaita:light ./toolbox
With this in mind, I create a "tla" alias in bash, by editing the .bashrc file, by entering the following line:
alias tla='GTK_THEME=Adwaita:light ~/TLA+/toolbox/toolbox'
Now when I open a terminal window, I can simply issue a "tla" command and it starts with a "light" scheme that is easy to read.