Use switch_theme as already indicated in another answer, but hook it to wpmu_new_blog
This function runs when a user self-registers a new site as well as when a super admin creates a new site. hook to ‘wpmu_new_blog’ for events that should affect all new sites.
https://codex.wordpress.org/function_reference/wpmu_create_blog
If not the code will run constantly, which will be inefficient and make switching themes impossible ( or unpredictable ).
In other words:
function default_theme_wpse_193024() {
switch_theme('twentythirteen');
}
add_action('wpmu_new_blog','default_theme_wpse_193024');
For switch_theme(), despite the parameter being named somewhat inexplicably “stylesheet”, use the name of the theme folder. Per the Codex:
Accepts one argument: $stylesheet of the theme. ($stylesheet is the
name of your folder slug. It’s the same value that you’d use for a
child theme, something liketwentythirteen.)