Some email clients (e.g. thdbd) have stripes in their summary buffer. That is, odd lines are a little shaded, for sake of readability.

Here is how to get a striped summary buffer in Gnus. Intensity of the shade can be increased or decreased by changing the value of the stripe-intensity variable. If that variable is nil, then a gray80 color will be used instead of computing the color of the stripes from the frame background color.

The coloring does not handle threading very well, so only the mail buffers are striped by default.

  ;; the color of the stripes is obtained by dimming the frame background color
  (defvar stripe-intensity 12
    "*intensity of the shade. Used to compute the color of the stripes.
     0 means no shading of the background color, nil means gray80")
  ;; a command that computes the rgb code of the shaded background color
  (defun shade-color (intensity)
  "print the #rgb color of the background, dimmed according to intensity"
  (interactive "nIntensity of the shade : ")
  (apply 'format "#%02x%02x%02x" 
     (mapcar (lambda (x)
       (if (> (lsh x -8) intensity)
           (- (lsh x -8) intensity)
      (color-values (cdr (assoc 'background-color (frame-parameters)))))))
  ;; the command that actually puts the stripes in the current buffer
  (defun stripe-alternate ()
   "stripes all down the current buffer"
   ;; compute the color of the stripes from the value of stripe-intensity
   (if stripe-intensity 
       (setq stripe-overlay-face (shade-color stripe-intensity))
     (setq stripe-overlay-face "gray80"))
   ;; put the overlay in the current buffer
    (goto-char (point-min))
    (let (stripe-overlay)
     (while (not (eobp))
       (setq stripe-overlay 
             (make-overlay (line-beginning-position)
                           (line-beginning-position 2)))
       (overlay-put stripe-overlay 'face (list :background stripe-overlay-face))
       (overlay-put stripe-overlay 'priority -1)
  ;; activate the stripes for the mail buffers only
  (add-hook 'gnus-summary-prepare-hook (lambda ()
      (with-current-buffer gnus-summary-buffer 
       (unless (gnus-news-group-p gnus-newsgroup-name)   

If you want stripes in all summary buffers, not just in mail buffers, delete the line (unless (gnus-news-group-p gnus-newsgroup-name) and remove one parenthesis at the very end of the last function.