Way to check if we are in Theme Customizer mode? [duplicate]

Question

Possible Duplicate:
How to execute conditional script when on new customize.php (Theme Customize) screen

Is there a Conditional Tag, or any other method, that we can use to check if we are customizing the theme?

I want to include some extra stylesheet while customizing the theme.

so I'd like to have sth like:

if ( theme-customizer-is-active ) {
    // load some extra styles
}