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/pkgs/theme/default.nix

@@ -0,0 +1,14 @@
+{ pkgs, depot, ... }:
+
+pkgs.callPackage
+  ({ emacsPackages }:
+  emacsPackages.trivialBuild {
+    pname = "theme";
+    version = "1.0.0";
+    src = ./theme.el;
+    packageRequires =
+      (with depot.users.wpcarro.emacs.pkgs; [
+        cycle
+      ]);
+  })
+{ }