You can add styles directly via the acf_register_block_type() function like so:
acf_register_block_type( [
'name' => '',
'title' => '',
'description' => '',
'render_callback' => '',
'category' => 'formatting',
'icon' => '',
'mode' => '',
'keywords' => [],
'supports' => [],
'styles' => [
[
'name' => 'light',
'label' => __('Light', 'abc'),
'isDefault' => true,
],
[
'name' => 'medium',
'label' => __('Medium', 'abc'),
],
[
'name' => 'dark',
'label' => __('Dark', 'abc'),
]
],
]);