OK, I have a bit more info on this. I thought it might be a config default or something, so I blew away Modrana (including the stuff left over after an uninstall) and then reinstalled it. That didn't help.
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However, I realized that it has to do with a usage pattern. If you select route->address to address and then click on "start" with the keyboard closed, you'll get the on-screen keyboard and you can see what you're typing with no problem. But if you open the keyboard, you'll get a little dialog window at the bottom fo the screen that reads "Input the start address". That one uses a black font on a black background. You can see the text if you swipe to select it, which changes the background to light blue.

I don't see anything in user_config.conf to control font colors. I don't have a .gtkrc controlling anything. However, after a bit of exploring, I found that I could modify mod_textEntry.py to fix the problem. After the line:
I added the following hack:
Of course, that's just a hack; it's (almost) as bad to have the color forced to white as to have it forced to black, since it won't work on light-colored themes. The proper solution would be to choose a color based on (1) the theme or user default, if specified; (2) white, if the overall brightness of the background is dark, or (3) black, if the overall brightness is light. But I'm not a gtk honcho, so I don't know how to write that code.

Hopefully, this post will give somebody who knows more about gtk what they need to fix this bug.