Fix Warning “Also define the standard property ‘box-shadow’ for compatibility”

It means you should add the standard style rule (box-shadow) in addition to the vendor prefixed version (-webkit-box-shadow)

.scrollbar-black::-webkit-scrollbar-thumb {
    border-radius: 10px;
    -webkit-box-shadow: inset 0 0 6px rgba(0, 0, 0, 0.1);
            box-shadow: inset 0 0 6px rgba(0, 0, 0, 0.1); // <- Add this to fix.    
    background-color: #000;
}