Thanks for reporting this. I'm sorry but I'm trying to find the behavior you are describing. Do you mean the toolbar of the wysiwyg editor? If so, I have checked a few sites and the text/background contrast appears normally. Can you give more details such as what browser you are using?
The Darkroom theme included in the Tiki package has a dark background; also there are a few themes available from this site that have dark backgrounds. All should work correctly for the release they are indicated for.