I've taken the liberty to modify your package a bit -- your unquotes
were wrong -- and move it to gnome-xyz.scm, the file I've added for
themes and extensions.  I verified, that the package builds and also
ran the linter.  The only errors are some versioning stuff (a false
positive) and a missing Software Heritage archive (which was to be
expected).

I noticed, that there are also some other themes inside of gnome.scm,
since my own file is rather fresh.  If the contributors of said
packages are reading this, I'd like them to also move their themes and
extensions to gnome-xyz.scm -- except adwaita-icon-theme, gnome-shell-
extensions, etc. which are part of GNOME itself. 

Thank you and best regards,

Leo