Lift restriction for finding theme summary line

* lisp/cus-theme.el (custom-theme-summary): Don't limit the file to
having the deftheme form as the very first form, rather look for the
deftheme form explicitly.

lisp/cus-theme.el

@@ -657,10 +657,12 @@ Theme files are named *-theme.el in `"))
 	    (insert-file-contents fn)
 	    (let ((sexp (let ((read-circle nil))
 			  (condition-case nil
-			      (read (current-buffer))
-			    (end-of-file nil)))))
-              (and (eq (car-safe sexp) 'deftheme)
-		   (setq doc (nth 2 sexp))))))))
+                              (progn
+                                (re-search-forward "^(deftheme")
+                                (beginning-of-line)
+                                (read (current-buffer)))
+                            (error nil)))))
+              (setq doc (nth 2 sexp)))))))
     (cond ((null doc)
 	   "(no documentation available)")
 	  ((string-match ".*" doc)