Hi,
as always in similar cases, it requests custom CSS. I can give you only basic tips, but I will not teach you CSS here.
1) This message comes from a language phrase (Solidres language files, not ours)
SR_RESERVATION_COMPLETE_REQUIRE_APPROVAL
2) It's displayed by this file:
components\com_solidres\controllers\reservation.php
3) Probably by default it uses bootstrap3 message box, where CSS class is named: alert alert-success
<div class="alert alert-success">....
so you have to override this style.
