You’ve also go the wrong hook
<?php
function acf_mod_styles() {
?>
<style type="text/css">
.acf-repeater.-row>table>tbody>tr+tr>td,
.acf-repeater.-block>table>tbody>tr+tr>td {
border-top-color: #a8a8a8; // about twice as dark
}
</style>
<?php
}
add_action('acf/input/admin_head', 'acf_mod_styles');
?>