WordPress at a glance
filter is not described

pre_set_theme_mod_(name) filter-hook . WP 3.9.0

Filters the theme mod value on save.

The dynamic portion of the hook name, $name, refers to the key name of the modification array. For example, 'header_textcolor', 'header_image', and so on depending on the theme options.

Usage

add_filter( 'pre_set_theme_mod_(name)', 'filter_function_name_456', 10, 2 );
function filter_function_name_456( $value, $old_value ){
	// filter...

	return $value;
}
$value(string)
The new value of the theme mod.
$old_value(string)
The current value of the theme mod.

Where the hook is called

set_theme_mod()
wp-includes/theme.php 904
$mods[ $name ] = apply_filters( "pre_set_theme_mod_{$name}", $value, $old_value );

Where the hook is used (in WP core)

Does not used.