For some bizarre reason CLR Browser in OBS will reset any changes you make on exit if any double quotes are used. You need to use single quotes instead. Here is the fix to your CSS
::-webkit-scrollbar {
visibility: hidden;
}
body {
background-color: rgba(0, 0, 0, 0);
margin: 0px auto;
}
#donor-notification {
background-image: url('http://db.deep.sg/ChequeEUR.png') !important;
}