A port of an old theme I had on my 3DS from the now defunct 3dsthem.es website.
It's one of my favorites so I wanted it on DSi too.
The original appears to have been made by a user named BlakCake.

A link to the original 3DS theme on the Internet Archive:
https://ia800807.us.archive.org/view_archive.php?archive=/20/items/3dsthemearchive/%5B7888%5D%20Relaxing%20Space%20by%20BlakCake.zip