Emacs符号的值作为变量是无效的

我的emacs init文件中有以下代码 -

(add-to-list 'custom-theme-load-path "~/.emacs.d/themes/solarized")
(load-theme 'solarized-light t)

我收到以下错误 -

Symbol's value as variable is void: custom-theme-load-path

回溯的错误是 -

Debugger entered--Lisp error: (void-variable custom-theme-load-path)
  add-to-list(custom-theme-load-path "~/.emacs.d/themes/solarized")
  eval-buffer(# nil "/home/kshitiz/.emacs" nil t)  ; Reading at buffer position 69
  load-with-code-conversion("/home/kshitiz/.emacs" "/home/kshitiz/.emacs" t t)
  load("~/.emacs" t t)
  #[nil "\205\264

我究竟做错了什么？