How do I change the highlight color for selected text with Emacs / deftheme?

I'm using Emacs 24; I've installed the zenburn theme, which is great, except I cannot see the selection highlight easily with the highlight color provided by zenburn:

By "selection" color, I mean the color of text that I've selected by setting a mark (C-space and moving the cursor to select text).

For the life of me, I cannot figure out how to change it. I've tried changing every combination of highlight, selection, etc.. that I can think of in zenburn-theme.el, but nothing seems to change it.

*For sanity's sake, I've tried changing other colors in the theme to make sure Emacs is loading the file properly - it is - those changes work.

I would have especially thought that changing highlight would work, but no customizations to the highlight line seem to work:

;;;; Built-in                                                                                                                                                                            
;;;;; basic coloring                                                 
   ...                                                                                                      
   `(highlight ((t (:background ,zenburn-bg-05 :foreground ,zenburn-yellow))))  

How can I change the selection color?

What you're looking for is the region face. For example:

(set-face-attribute 'region nil :background "#666")

In addition to tungd's solution. You could also then change the font color to make it more readable. E.g. to white

(set-face-attribute 'region nil :background "#666" :foreground "#ffffff")

You can use the customize interface.

M-x customize-face

When prompted for which face, enter region. Then you'll see something like:

Using customize makes it easy to experiment with lots of colors. Try clicking [ Choose ] to access the color-picker), then [ Apply and Save ]. It also nicely organizes your customizations into a single file (rather than further polluting your init.el).