If you want to write a theme, please see the documentation at:
Themes are installed and packaged as extensions - once installed, the theme specification is copied into your browser preferences, allowing users to override individual theme choices. This theme data is loaded and managed by browser_theme_provider.cc (which also handles telling everyone to repaint when a theme is applied).
The theme provider has the following interface:
This wraps ResourceBundle::GetBitmapNamed and provides themed images if they exist, falling back to the default ResourceBundle images otherwise.
Returns an SkColor with the specified ID (see the list of ids in browser_theme_provider.h)
All views have access to a theme provider through their GetThemeProvider() method - this returns the root widget's theme provider. In browser-ui land, the theme provider hangs off the profile, and the root widget is profile aware.
In some cases, the root widget is not profile aware, and so you will have to inject the theme provider into the view. An example of this is tab_renderer.cc, which doesn't have access to the profile while in its dragged state.