set_theme_mod with array

Hello,

I need to manually update a theme mod (or customize value) when it is in an array.

This works:
set_theme_mod( 'myvar' , 'my value' ) ;

But how to set the value when it is in array?
non of these work, after hours of searching I cannot seem to find an answer.

set_theme_mod( 'myarray["myvar"]' , 'my value' ) ;
set_theme_mod( 'myarray' , array('myvar'=>'my value' ) ;

The only hint I can get is that when calling the value from an array you use.

$options = get_option('myarray');
echo $options['myvar']);

for single you would use get_theme_mod("myvar");