How to add arrows with -webkit-scrollbar-button

Here's a light version.

::-webkit-scrollbar {
    width: 16px;
    height: 16px;
}

::-webkit-scrollbar-corner,
::-webkit-scrollbar-track {
    background-color:#eee;
}

::-webkit-scrollbar-thumb {
    background-color: #8f8e8e;
    background-clip: padding-box;
    border: 2px solid transparent;
}

::-webkit-scrollbar-thumb:hover {
    background-color: rgb(112, 112, 112);
}

::-webkit-scrollbar-thumb:active {
    background-color: rgb(128, 128, 128);
}

/* Buttons */
::-webkit-scrollbar-button:single-button {
    background-color: #eee;
    display: block;
    background-size: 10px;
    background-repeat: no-repeat;
}

/* Up */
::-webkit-scrollbar-button:single-button:vertical:decrement {
    height: 12px;
    width: 16px;
    background-position: center 4px;
    background-image: url("data:image/svg+xml;utf8,<svg xmlns='http://www.w3.org/2000/svg' width='100' height='100' fill='rgb(73, 73, 73)'><polygon points='50,00 0,50 100,50'/></svg>");
}

::-webkit-scrollbar-button:single-button:vertical:decrement:hover {
    background-image: url("data:image/svg+xml;utf8,<svg xmlns='http://www.w3.org/2000/svg' width='100' height='100' fill='rgb(112, 112, 112)'><polygon points='50,00 0,50 100,50'/></svg>");
}

::-webkit-scrollbar-button:single-button:vertical:decrement:active {
    background-image: url("data:image/svg+xml;utf8,<svg xmlns='http://www.w3.org/2000/svg' width='100' height='100' fill='rgb(128, 128, 128)'><polygon points='50,00 0,50 100,50'/></svg>");
}

/* Down */
::-webkit-scrollbar-button:single-button:vertical:increment {
    height: 12px;
    width: 16px;
    background-position: center 2px;
    background-image: url("data:image/svg+xml;utf8,<svg xmlns='http://www.w3.org/2000/svg' width='100' height='100' fill='rgb(73, 73, 73)'><polygon points='0,0 100,0 50,50'/></svg>");
}

::-webkit-scrollbar-button:single-button:vertical:increment:hover {
    background-image: url("data:image/svg+xml;utf8,<svg xmlns='http://www.w3.org/2000/svg' width='100' height='100' fill='rgb(112, 112, 112)'><polygon points='0,0 100,0 50,50'/></svg>");
}

::-webkit-scrollbar-button:single-button:vertical:increment:active {
    background-image

A newer version and better is this: How do I switch to Chromes dark scrollbar like GitHub does?

here is complete scrollbar with buttons with dark mode:

https://codepen.io/patrikx3/pen/ZEBQQyV

::-webkit-scrollbar {
    width: 16px;
    height: 16px;
}

::-webkit-scrollbar-corner,
::-webkit-scrollbar-track {
    background-color: rgb(64, 64, 64);
}

::-webkit-scrollbar-thumb {
    background-color: rgb(96, 96, 96);
    background-clip: padding-box;
    border: 2px solid transparent;
}

::-webkit-scrollbar-thumb:hover {
    background-color: rgb(112, 112, 112);
}

::-webkit-scrollbar-thumb:active {
    background-color: rgb(128, 128, 128);
}

/* Buttons */
::-webkit-scrollbar-button:single-button {
    background-color: rgb(64, 64, 64);

    display: block;
    background-size: 10px;
    background-repeat: no-repeat;
}

/* Up */
::-webkit-scrollbar-button:single-button:vertical:decrement {
    height: 12px;
    width: 16px;
    background-position: center 4px;
    background-image: url("data:image/svg+xml;utf8,<svg xmlns='http://www.w3.org/2000/svg' width='100' height='100' fill='rgb(96, 96, 96)'><polygon points='50,00 0,50 100,50'/></svg>");
}

::-webkit-scrollbar-button:single-button:vertical:decrement:hover {
    background-image: url("data:image/svg+xml;utf8,<svg xmlns='http://www.w3.org/2000/svg' width='100' height='100' fill='rgb(112, 112, 112)'><polygon points='50,00 0,50 100,50'/></svg>");
}

::-webkit-scrollbar-button:single-button:vertical:decrement:active {
    background-image: url("data:image/svg+xml;utf8,<svg xmlns='http://www.w3.org/2000/svg' width='100' height='100' fill='rgb(128, 128, 128)'><polygon points='50,00 0,50 100,50'/></svg>");
}

/* Down */
::-webkit-scrollbar-button:single-button:vertical:increment {
    height: 12px;
    width: 16px;
    background-position: center 2px;
    background-image: url("data:image/svg+xml;utf8,<svg xmlns='http://www.w3.org/2000/svg' width='100' height='100' fill='rgb(96, 96, 96)'><polygon points='0,0 100,0 50,50'/></svg>");
}

::-webkit-scrollbar-button:single-button:vertical:increment:hover {
    background-image: url("data:image/svg+xml;utf8,<svg xmlns='http://www.w3.org/2000/svg' width='100' height='100' fill='rgb(112, 112, 112)'><polygon points='0,0 100,0 50,50'/></svg>");
}

::-webkit-scrollbar-button:single-button:vertical:increment:active {
    background-image: url("data:image/svg+xml;utf8,<svg xmlns='http://www.w3.org/2000/svg' width='100' height='100' fill='rgb(128, 128, 128)'><polygon points='0,0 100,0 50,50'/></svg>");
}

/* Left */
::-webkit-scrollbar-button:single-button:horizontal:decrement {
    height: 12px;
    width: 12px;
    background-position: 3px 3px;
    background-image: url("data:image/svg+xml;utf8,<svg xmlns='http://www.w3.org/2000/svg' width='100' height='100' fill='rgb(96, 96, 96)'><polygon points='0,50 50,100 50,0'/></svg>");

}

::-webkit-scrollbar-button:single-button:horizontal:decrement:hover {
    background-image: url("data:image/svg+xml;utf8,<svg xmlns='http://www.w3.org/2000/svg' width='100' height='100' fill='rgb(112, 112, 112)'><polygon points='0,50 50,100 50,0'/></svg>");
}

::-webkit-scrollbar-button:single-button:horizontal:decrement:active {
    background-image: url("data:image/svg+xml;utf8,<svg xmlns='http://www.w3.org/2000/svg' width='100' height='100' fill='rgb(128, 128, 128)'><polygon points='0,50 50,100 50,0'/></svg>");
}

/* Right */
::-webkit-scrollbar-button:single-button:horizontal:increment {
    height: 12px;
    width: 12px;
    background-position: 3px 3px;
    background-image: url("data:image/svg+xml;utf8,<svg xmlns='http://www.w3.org/2000/svg' width='100' height='100' fill='rgb(96, 96, 96)'><polygon points='0,0 0,100 50,50'/></svg>");
}

::-webkit-scrollbar-button:single-button:horizontal:increment:hover {
    background-image: url("data:image/svg+xml;utf8,<svg xmlns='http://www.w3.org/2000/svg' width='100' height='100' fill='rgb(112, 112, 112)'><polygon points='0,0 0,100 50,50'/></svg>");
}

::-webkit-scrollbar-button:single-button:horizontal:increment:active {
    background-image: url("data:image/svg+xml;utf8,<svg xmlns='http://www.w3.org/2000/svg' width='100' height='100' fill='rgb(128, 128, 128)'><polygon points='0,0 0,100 50,50'/></svg>");
}

(Super late to the party, but still)

You can style scrollbar buttons using ::-webkit-scrollbar-button selector (see this blog post for a full breakdown on the webkit scrollbar pseudo selectors), but getting them to show arrows is much trickier, and most people seem to either use background images or skip buttons altogether.

Here's a solution using CSS triangles for arrows:

https://stackoverflow.com/a/46229219/1202275

and another one (based on the one above, vertical scrollbar only but the idea is the same):

::-webkit-scrollbar {
  width: 16px;
  border: 5px solid white;

}

::-webkit-scrollbar-thumb {
  background-color: #b0b0b0;
  background-clip: padding-box;
  border: 0.05em solid #eeeeee;
}

::-webkit-scrollbar-track {
  background-color: #bbbbbb;
}
/* Buttons */
::-webkit-scrollbar-button:single-button {
  background-color: #bbbbbb;
  display: block;
  border-style: solid;
  height: 13px;
  width: 16px;
}
/* Up */
::-webkit-scrollbar-button:single-button:vertical:decrement {
  border-width: 0 8px 8px 8px;
  border-color: transparent transparent #555555 transparent;
}

::-webkit-scrollbar-button:single-button:vertical:decrement:hover {
  border-color: transparent transparent #777777 transparent;
}
/* Down */
::-webkit-scrollbar-button:single-button:vertical:increment {
  border-width: 8px 8px 0 8px;
  border-color: #555555 transparent transparent transparent;
}

::-webkit-scrollbar-button:vertical:single-button:increment:hover {
  border-color: #777777 transparent transparent transparent;
}

(https://codepen.io/DarthVeyda/pen/eLLbXa)