What Is Scrollbar Thumb Height In CSS?

the height of the thumb is based in the size of content, you can change the width inside the ::-webkit-scrollbar but the height will always be based on the content.

::-webkit-scrollbar { /* 1 */ } 
::-webkit-scrollbar-button { /* 2 */ } 
::-webkit-scrollbar-track { /* 3 */ } 
::-webkit-scrollbar-track-piece { /* 4 */ } 
::-webkit-scrollbar-thumb { /* 5 */ }
 ::-webkit-scrollbar-corner { /* 6 */ } 
::-webkit-resizer { /* 7 */ }

Customized WebKit Scrollbar

::-webkit-scrollbar {
    width: 12px;
}
 
/* Track */
::-webkit-scrollbar-track {
    -webkit-box-shadow: inset 0 0 6px rgba(0,0,0,0.3); 
    -webkit-border-radius: 10px;
    border-radius: 10px;
}
 
/* Handle */
::-webkit-scrollbar-thumb {
    -webkit-border-radius: 10px;
    border-radius: 10px;
    background: rgba(255,0,0,0.8); 
    -webkit-box-shadow: inset 0 0 6px rgba(0,0,0,0.5); 
}
::-webkit-scrollbar-thumb:window-inactive {
	background: rgba(255,0,0,0.4); 
}