Check out example codes for "css edit scroll bar". It will help you in understanding the concepts better.

Code Example 1

::-webkit-scrollbar{width:6px;border-left:1px solid #E6ECF8;}
::-webkit-scrollbar-thumb{background-color:#d6872c;}

Code Example 2

/* width */
*::-webkit-scrollbar {
  width: 10px;
}

/* Track */
*::-webkit-scrollbar-track {
  background: #f1f1f1; 
}
 
/* Handle */
*::-webkit-scrollbar-thumb {
  background: #888; 
}

/* Handle on hover */
*::-webkit-scrollbar-thumb:hover {
  background: #555; 
}

Code Example 3

::-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 */ }


/* Different States */
:horizontal
:vertical
:decrement
:increment
:start
:end 
:double-button
:single-button
:no-button
:corner-present
:window-inactive


/* All toghether example */
::-webkit-scrollbar-track-piece:start {
   /* Select the top half (or left half) or scrollbar track individually */
}

::-webkit-scrollbar-thumb:window-inactive {
   /* Select the thumb when the browser window isn't in focus */
}

::-webkit-scrollbar-button:horizontal:decrement:hover {
   /* Select the down or left scroll button when it's being hovered by the mouse */
}


/* Example */
body::-webkit-scrollbar {
    width: 1em;
}
body::-webkit-scrollbar-track {
    -webkit-box-shadow: inset 0 0 6px rgba(0,0,0,0.3);
}
body::-webkit-scrollbar-thumb {
  background-color: darkgrey;
  outline: 1px solid slategrey;
}

Code Example 4

/* Answer to: "custom scrollbar css" */

#yourContainer::-webkit-scrollbar {
	width: 12.34px;
	/* You can add other properties & values */
}

#yourContainer::-webkit-scrollbar-track {
	background-color: #123456;
	/* You can add other properties & values */
}

#yourContainer::-webkit-scrollbar-thumb {
	background: #123456;
    outline: 1px solid #654321;
	/* You can add other properties & values */
}

/* To customize the default scrollbar, just do this: */
 ::-webkit-scrollbar { /* blah blah blah */ }
/* Notice how there's no selector ^^ */

/* Other things to note: */

::-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 */ }


/* Different States */
:horizontal
:vertical
:decrement
:increment
:start
:end 
:double-button
:single-button
:no-button
:corner-present
:window-inactive

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