Eternity was a theme for 3.9 that tried to emulate Beatmania IIDX . The 5.0.12 rebuild took it further. It utilized "Runtime Effects" to blur the background video based on your combo multiplier. This feature was deprecated in 5.1. If you want that specific visual blur, you cannot upgrade.
: A custom modification of the default theme that adds a blue color scheme, unique FC (Full Combo) animations, a "Warning Screen" upon launch, and a DDR 2014-style scrolling music wheel. DDR A (Zenius-I-vanisher) : A faithful recreation of the Dance Dance Revolution A stepmania 5012 themes exclusive
Ensure you are not putting the theme in the "Data" folder. Eternity was a theme for 3