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
+      ]);
+  })
+{ }

users/wpcarro/emacs/pkgs/theme/theme.el

@@ -0,0 +1,78 @@
+;;; theme.el --- Colors and stuff -*- lexical-binding: t -*-
+
+;; Author: William Carroll <wpcarro@gmail.com>
+;; Version: 0.0.1
+;; Package-Requires: ((emacs "24.3"))
+
+;;; Commentary:
+;;
+;; Cycle through a whitelist of themes.
+
+;;; Code:
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Dependencies
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(require 'cycle)
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Library
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(defgroup theme nil
+  "Customization options for `theme'."
+  :group 'theme)
+
+(defcustom theme-whitelist
+  (cycle-from-list (custom-available-themes))
+  "The whitelist of themes through which to cycle."
+  :type '(cycle symbol)
+  :group 'theme)
+
+(defcustom theme-after-change
+  nil
+  "Hook invoked after a new theme is loaded"
+  :type 'hook
+  :group 'theme)
+
+(defun theme-whitelist-set (theme)
+  "Focus the THEME in the `theme-whitelist' cycle."
+  (cycle-focus! (lambda (x) (equal x theme)) theme-whitelist)
+  (theme--set (cycle-current theme-whitelist)))
+
+(defun theme-select ()
+  "Load a theme using `completing-read'."
+  (interactive)
+  (let ((theme (completing-read "Theme: " (cycle-to-list theme-whitelist))))
+    (theme--disable-all)
+    (theme--set (intern theme))))
+
+(defun theme-next ()
+  "Disable the currently active theme and load the next theme."
+  (interactive)
+  (disable-theme (cycle-current theme-whitelist))
+  (theme--set (cycle-next! theme-whitelist))
+  (message (format "Active theme: %s" (cycle-current theme-whitelist))))
+
+(defun theme-prev ()
+  "Disable the currently active theme and load the previous theme."
+  (interactive)
+  (disable-theme (cycle-current theme-whitelist))
+  (theme--set (cycle-prev! theme-whitelist))
+  (message (format "Active theme: %s" (cycle-current theme-whitelist))))
+
+(defun theme--disable-all ()
+  "Disable all currently enabled themes."
+  (interactive)
+  (dolist (x custom-enabled-themes)
+    (disable-theme x)))
+
+(defun theme--set (theme)
+    "Call `load-theme' with `THEME', ensuring that the line numbers are bright.
+There is no hook that I'm aware of to handle this more elegantly."
+    (load-theme theme t)
+    (run-hooks 'theme-after-change))
+
+(provide 'theme)
+;;; theme.el ends here