This isn't possible. Glancing at this source code shows what parts of Chrome are themeable:
// The image resources we will allow people to theme.
const int kThemeableImages[] = {
IDR_THEME_FRAME,
IDR_THEME_FRAME_INACTIVE,
IDR_THEME_FRAME_INCOGNITO,
IDR_THEME_FRAME_INCOGNITO_INACTIVE,
IDR_THEME_TOOLBAR,
IDR_THEME_TAB_BACKGROUND,
IDR_THEME_TAB_BACKGROUND_INCOGNITO,
IDR_THEME_TAB_BACKGROUND_V,
IDR_THEME_NTP_BACKGROUND,
IDR_THEME_FRAME_OVERLAY,
IDR_THEME_FRAME_OVERLAY_INACTIVE,
IDR_THEME_BUTTON_BACKGROUND,
IDR_THEME_NTP_ATTRIBUTION,
IDR_THEME_WINDOW_CONTROL_BACKGROUND
};
You can see that the address box isn't present in the list and consequently, cannot be changed.