In WordPress 3.5, this hook works for me:
add_action( 'print_media_templates', 'wpse_75746_print_style_35' );
function wpse_75746_print_style_35()
{
?>
<style>
.media-modal-content, .media-sidebar {
background: #FFF2D4 !important;
}
</style>
<?php
}
In 3.4.2, this is the one:
add_action( 'admin_print_styles-media-upload-popup', 'wpse_75746_print_style_342' );
function wpse_75746_print_style_342()
{
?>
<style>
#media-upload {
background: #FFF2D4 !important;
}
</style>
<?php
}