How to develop a Google chrome theme?

Download any available theme for Chrome, like this one for instance.

Rename it so it has a .zip extension. Open it with your favorite archiving tool.

The theme is described in a file called manifest.json. It gives links to images used in the theme. They're in the "i" directory in the Greyscale one. The images are in PNG format.

Here is the format of the manifest.json file for Greyscale:

{
"version":"1.0",
"name":"Greyscale",
"theme":
{"images":
    {
    "theme_frame":"i/agxjaHJvbWV0aGVtZXNyDAsSBEZpbGUY6ZwBDA",
    "theme_toolbar":"i/agxjaHJvbWV0aGVtZXNyDAsSBEZpbGUY6pwBDA",
    "theme_button_background":"i/agxjaHJvbWV0aGVtZXNyDAsSBEZpbGUYtawBDA"
    },
"colors":
    {
    "frame":[32,32,32],
    "toolbar":[210,210,246],
    "tab_text":[0,0,0],
    "tab_background_text":[0,0,0],
    "bookmark_text":[0,0,0],
    "ntp_background":[235,235,235],
    "ntp_text":[0,0,0],
    "ntp_link":[0,0,120],
    "ntp_section":[210,210,210,1],
    "ntp_section_text":[0,0,0],
    "ntp_section_link":[0,0,120]
    },
"properties":
    {
    "ntp_background_alignment":"bottom",
    "ntp_background_repeat":"no-repeat"},
    "tints":
        {
        "buttons":[0.6,0,0.5]
        }
    }
}

Use this as a template for your own themes.

You'll find more information here too.

There is an official guide to creating themes on Chromium’s Google Code site.