core: add support for :location keyword in dotspacemacs-themes

New functions:
- configuration-layer/get-location-directory which return the location on disk
given a location
- spacemacs//get-theme-directory which returns the location on disk of the
theme

Add note in documentation to warn about the directory name when :location local
is used, the directory name is the package name not the theme name.

core/core-configuration-layer.el

@@ -1576,25 +1576,12 @@ wether the declared layer is an used one or not."
         (spacemacs-buffer/message (format "%S is toggled off." pkg-name)))
        (t
         ;; load-path
-        (let ((location (oref pkg :location)))
-          (cond
-           ((stringp location)
-            (if (file-directory-p location)
-                (push (file-name-as-directory location) load-path)
-              (configuration-layer//warning
-               "Location path for package %S does not exists (value: %s)."
-               pkg location)))
-           ((and (eq 'local location)
-                 (eq 'dotfile (car (oref pkg :owners))))
-            (push (file-name-as-directory
-                   (concat configuration-layer-private-directory "local/"
-                           (symbol-name (oref pkg :name))))
-                  load-path))
-           ((eq 'local location)
-            (let* ((owner (configuration-layer/get-layer
-                           (car (oref pkg :owners))))
-                   (dir (when owner (oref owner :dir))))
-              (push (format "%slocal/%S/" dir pkg-name) load-path)))))
+        (let ((dir (configuration-layer/get-location-directory
+                    pkg-name
+                    (oref pkg :location)
+                    (car (oref pkg :owners)))))
+          (when dir
+            (add-to-list 'load-path dir)))
         ;; configuration
         (unless (memq (oref pkg :location) '(local site built-in))
           (configuration-layer//activate-package pkg-name))
@@ -1605,6 +1592,27 @@ wether the declared layer is an used one or not."
          (t
           (configuration-layer//configure-package pkg))))))))
 
+(defun configuration-layer/get-location-directory (pkg-name location owner)
+  "Return the location on disk for PKG."
+  (cond
+   ((stringp location)
+    (if (file-directory-p location)
+        (file-name-as-directory location)
+      (configuration-layer//warning
+       "Location path for package %S does not exists (value: %s)."
+       pkg-name location)
+      nil))
+   ((eq 'local location)
+    (let ((dir (if (eq 'dotfile owner)
+                   configuration-layer-private-directory
+                 (let* ((owner (configuration-layer/get-layer owner)))
+                   (when owner (oref owner :dir))))))
+      (if dir
+          (file-name-as-directory (format "%slocal/%S/" dir pkg-name))
+        (configuration-layer//warning
+         "Cannot find path location path for package %S." pkg-name)
+        nil)))))
+
 (defun configuration-layer//package-enabled-p (pkg layer)
   "Returns true if PKG should be configured for LAYER.
 LAYER must not be the owner of PKG."

core/core-themes-support.el

@@ -184,6 +184,20 @@ package name does not match theme name + `-theme' suffix.")
       (car theme)
     theme))
 
+(defun spacemacs//get-theme-package-directory (theme)
+  "Return the THEME location on disk."
+  (let* ((theme-name (spacemacs//get-theme-name theme))
+         (pkg-name (spacemacs/get-theme-package-name theme-name))
+         (dir (configuration-layer/get-location-directory
+               pkg-name
+               (plist-get (cdr theme) :location)
+               'dotfile)))
+    (unless dir
+      ;; fallback to elpa directory
+      (setq dir (configuration-layer/get-elpa-package-install-directory
+                 pkg-name)))
+    dir))
+
 (defun spacemacs/load-default-theme (&optional fallback-theme)
   "Load default theme.
 Default theme is the car of `dotspacemacs-themes'.
@@ -203,16 +217,11 @@ THEME."
           ;; Load theme
           (unless (or (memq theme-name (custom-available-themes))
                       (eq 'default theme-name))
-            (let* ((pkg-name (spacemacs/get-theme-package-name theme-name))
-                   (pkg-dir
-                    (when pkg-name
-                      (configuration-layer/get-elpa-package-install-directory
-                       pkg-name))))
-              ;; add theme package directory to load-path since `package.el' may
-              ;; not be initialized when theme is applied
+            (let ((pkg-dir (spacemacs//get-theme-package-directory theme)))
               (when pkg-dir
                 (add-to-list 'custom-theme-load-path pkg-dir)
                 (add-to-list 'load-path pkg-dir)
+                ;; do we still need this particual case for moe theme?
                 (when (or (eq 'moe-light theme-name)
                           (eq 'moe-dark theme-name))
                   (load-file (concat pkg-dir "moe-light-theme.el"))

doc/DOCUMENTATION.org

@@ -1026,6 +1026,13 @@ declaration:
               ))
 #+END_SRC
 
+*Important note:* If you use =:location local= then you have to put your theme
+in the directory =private/local/<theme-package-name>/= with =theme-package-name=
+being the name of your package suffixed with =-theme= as mentioned in the Emacs
+conventions.
+ For instance if your theme is =foo= then you have to put our theme
+files in the directory =private/local/foo-theme=.
+
 You can cycle between the themes declared in =dotspacemacs-themes= with
 ~SPC T n~ and select an installed theme with ~SPC T s~.