It is safe to ignore that message. It means there is a key in the style that isn’t used, likely because it was saved in an earlier version of the OmniFocusColors.app or it was exported from an earlier version of OmniFocus when we still used that key. If I remember correctly, we removed some keys because they were no longer used, so setting it wouldn’t make any visual difference.
The fields would only be black on black if either it is defined that way in the style, or if your accessibility settings are changing the contrast/color from what is defined. To check what colors are defined in the style, you can get it here and open up the style. To see if your settings are altering the specified colors, go to Settings, Accessibility, Display and see what you have set there. Normal with everything unchecked is least likely to interfere with the style as specified.
If you want to import that style without seeing the error, set your style to using that option in OmniFocus, then export it in Style preferences in the most recent build. The extra keys should be stripped out that way, or you can remove them yourself using an XML editor if you are feeling brave.
I hope these ideas help some! Glad you are enjoying the many creative styles that our artistic OmniFocus users have created. It is wonderful that people are willing to share their work so that we all benefit from more options.