Unfortunately, there's nothing I can do about that, because it's actually a bug in the contact app itself. (It uses the theme's ButtonTextColor on non-button objects, resulting in invisible text.) I could change the ButtonTextColor in the theme, but that would change the look of buttons everywhere, and the theme's current colors work fine with any correctly-built UI. I did file a bug which you could vote on, but Nokia has already said they'll only fix "critical" bugs in Maemo 5 at this point so I doubt this one will be resolved.

You could modify theme images on your device (they're in /opt/usr/share/themes/okuda/images/ and /opt/usr/share/icons/okuda/) but then you'd have to work on one image at a time, and you'd have no easy way to share your modifications.

The better option would be to download the theme source, make your changes in the SVG templates (they were made with Inkscape, but you could actually make batch color changes to the SVG files with search/replace in a text editor), export the modified SVGs as PNGs, and rebuild the theme. If you do this, be sure to look through all the text files in the source and change them as needed (add your name/email to the maintainer field, change the package name, theme name, etc.) I'd be happy to look over your modified source before you upload it to extras-devel if you want.