The modding community is resilient. While many links have faded over the years, there are two primary places where the community actively shares and preserves Aurora themes.
Here are the three verified locations where you can find active, safe download links. xbox 360 aurora themes link
The theme is incompatible with your Aurora version. Update Aurora to the latest beta (0.7b+). Or, manually edit theme.xml – change <version> to match your build. The modding community is resilient