The Onyx WordPress Theme has been updated to fix minor issues. These include a small html issue with the footer, and also fixing HTML validation errors that occurred inside single posts.
The new version is 1.0.4 and has been tested out on WordPress 3.1 and works well. In case you install it without reading any notices – this theme is designed to work with three widgets; these get displayed underneath the content, above the footer. If you add more widgets, they will get displayed in one of the three columns, but does not distribute evenly across the three.