Make an option to change all text color to basic colors.
When you tap the option, it will open a little window, kind of like the them color window. You could put a few colors like black, white, red, blue, and green.
At the very minimum, it would be really great to have the option to change the white text color to black. Black is MUCH more readable on most background colors. This can be seen by comparing Textra to stockApp and is the only thing that is stopping me switching to Textra.