Change theme_image_widget in other theme I have a site, where I want to alter the form of the image widget.

I have a admin theme and overlay activated. The Admin theme is set to the default 'seven' theme from the core (V7.14)

Now I want to write

function seven_image_widget($vars){
    $vars['element']['alt']['#title'] = 'Foo';
    return theme_image_widget($vars);
}


If I add this in the template.php of seven theme, it works, if I put it in my custom theme, it does not work.
Can I access this theme hook of seven theme from my theme in any way?  I don't want to change the core themes code.
I found it myself.
If made a subtheme of seven (named sevensub) with just this special function in template.php

function sevensub_image_widget($vars){
    $vars['element']['alt']['#title'] = 'Foo';
    return theme_image_widget($vars);
}