If you are using the latest version (ACF Pro 5.8.1) i think the following code will help
/* add border to the repeater block row */
.acf-repeater.-block > table > tbody > tr > td {
border-top-width: 5px;
border-top-color: #046f8a!important;
}
/* exclude the forist row */
.acf-repeater.-block > table > tbody > tr:first-child > td {
border-top-width: 0px;
border-top-color: #046f8a!important;
}
/* in case if you got any nested repeater blocks and want to hide the border */
.acf-repeater.-block > table > tbody > tr > td .acf-repeater.-block > table > tbody > tr > td {
border-top-width: 0px;
border-top-color: #E1E1E1;
}
enqueue these styles in the admin.
