I have beating my head around on this one. Where in the css is the moo popup text color defined. Currently it looks like it is using my link color which is just off black and with the black popup background it's next to impossible to read. On that same note does anyone know were the background color for the popup is defined?
Those are for regular mootips that you can use in your own code (stories, staticpages, in templates, etc.)
For forum specific mootips though, the settings were placed with the forum specific templates. Have a look in private/plugins/forum/css/style.css.
To save yourself from having to remember those changes on an upgrade, create public_html/layout/nouveau/plugins/forum/css/style.css and make your changes there, as the system will look there before going into the private/plugins/forum/css directory.