I've never actually downloaded one. They're quite simple to make (for R4 anyway). Essentially just four 256x192 bitmaps and an .ini file to assign colors. Filenames are as follows (for R4):

logo.bmp - Top screen main logo.
icons.bmp - Bottom screen main logo with the three main buttons.
bckgrd_1.bmp - Top screen secondary logo (file manager)
bckgrd_2.bmp - Bottom screen secondary logo (cheats/rom info)
theme.ini - Stores theme and color information.

Here's what a theme.ini might look like (without comments):


The 4 bitmaps and ini file reside in: /_system_/themes/themeXX/ (XX being 01-12 I think) when it comes to an R4. Hope this helped.