feat(theme): improve doom-themes overrides by using a proper theme

Instead of manually just setting faces after loading a doom-themes
theme, let's use a custom override theme which we apply right after
applying any doom-themes theme.

The override theme uses various doom-color helpers, so the colors it
uses will be based on the most recently applied doom-themes theme.

core/siren-core-init.el

@@ -60,6 +60,9 @@
 (setq custom-file (expand-file-name "custom.el" siren-dir))
 (load-file custom-file)
 
+;; Enable custom themes
+(require 'siren-core-themes)
+
 ;; The modules
 (require 'siren-core-modules)
 

core/siren-core-modules.el

@@ -10,14 +10,14 @@
   "Root directory for Emacs Siren modules.")
 (siren-recursive-add-to-load-path siren-modules-dir)
 
+;; Theme
+(require 'siren-doom-themes)
+
 ;; Core
 (require 'siren-aliases)
 (require 'siren-global-keybindings)
 (require 'siren-packages)
 
-;; Theme
-(require 'siren-doom-themes)
-
 ;; Completion
 (require 'siren-company)
 

core/siren-core-themes.el

@@ -0,0 +1,14 @@
+;;; siren-core-themes.el --- jimeh's Emacs Siren: Custom themes.
+
+;;; Commentary:
+
+;; Enable loading custom themes.
+
+;;; Code:
+
+(defvar siren-themes-dir (expand-file-name "themes" siren-dir)
+  "Root directory for Emacs Siren custom themes.")
+(add-to-list 'custom-theme-load-path siren-themes-dir)
+
+(provide 'siren-core-themes)
+;;; siren-core-themes.el ends here