feat(wpcarro/emacs): Package theme.el

**TL;DR:**
- Rename `colorscheme.el` to `theme.el` to align with Emacs's nomenclature.
- Prune dependencies:
  - `cl-lib`
  - `>`
  - `prelude`

Change-Id: I15f225671b4096ab08913583b7b464e316c95298
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7412
Tested-by: BuildkiteCI
Reviewed-by: wpcarro <wpcarro@gmail.com>

users/wpcarro/emacs/default.nix

@@ -53,6 +53,7 @@ let
       string
       struct
       symbol
+      theme
       tuple
       vterm-mgt
       zle