Note:
You need to media-query them, otherwise you kill the styled Chrome-scrollbars.
Also, you need to apply the scrollbars for the document on the html element, not the body-element.
Firefox-Scrollbars:
@-moz-document url-prefix()
{
html
{
scrollbar-color: #D2D2D2 #2E3645;
scrollbar-width: thin; /* auto, thin, none*/
}
:root
{
scrollbar-color: rgb(210,210,210) rgb(46,54,69) !important;
scrollbar-width: thin !important;
}
}
Chrome-Scrollbars
* {
scrollbar-arrow-color: #FFA500;
scrollbar-base-color: black;
scrollbar-dark-shadow-color: #aaaaaa;
scrollbar-track-color: black;
scrollbar-face-color: #3d3d3d;
scrollbar-shadow-color: gray;
scrollbar-highlight-color: silver;
scrollbar-3d-light-color: black;}
::-webkit-scrollbar{width: 13px; height: 13px}
::-webkit-scrollbar:hover{height: 18px}
::-webkit-scrollbar-button:start:decrement,
::-webkit-scrollbar-button:end:increment{
height: 15px;
width: 13px;
display: block;
background: #101211;
background-repeat: no-repeat}
::-webkit-scrollbar-button:horizontal:decrement{
background-image: url('data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAcAAAAHCAMAAADzjKfhAAAAGXRFWHRTb2Z0d2FyZQBBZG9iZSBJbWFnZVJlYWR5ccllPAAAADlQTFRFEBIREhQSHSAWIiUYJioZNz0gPkQjQEcjRUwlSVAnXWYuaHIyanQziJY/mKdFq7xMssRPw9dVyt5YQCZ/wAAAACtJREFUCB0FwQECgBAQALBFlKi4/z/WBlKD45kJ1x8DdUXccL7RgfJlQLEBGtwA6QSuDq4AAAAASUVORK5CYII=');
background-position: 4px 3px}
::-webkit-scrollbar-button:horizontal:increment{
background-image: url('data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAcAAAAHCAMAAADzjKfhAAAAGXRFWHRTb2Z0d2FyZQBBZG9iZSBJbWFnZVJlYWR5ccllPAAAADNQTFRFEBIREhQSHSAWJioZNz0gPEIiQkkkRUwlR04mYWswanQziJY/mKdFq7xMssRPw9dVyt5Ye+rBnQAAACpJREFUCB0FwYkBgCAQACD8U9Nu/2kDKqC8FVixC4yIr2HF6ch3JqQnww8VPADN4gkr7wAAAABJRU5ErkJggg==');
background-position: 3px 3px}
::-webkit-scrollbar-button:vertical:decrement{
background-image: url('data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAcAAAAHCAIAAABLMMCEAAAAGXRFWHRTb2Z0d2FyZQBBZG9iZSBJbWFnZVJlYWR5ccllPAAAAHFJREFUeNpiFBASZAADPSOhS+feQdhMEEpEjKN5gpGRuTCKaEqeGicXc2q+OhMTI1RUx0DQyUMSyFBW4/UOkgEyWICSCVkqS2bfvX/ns7wST3C0woFdLxiNLeQ+fvj95tUPiFHcPCxAaxnhbkAGAAEGAAxaGY5HzMiTAAAAAElFTkSuQmCC');
background-position: 3px 4px}
::-webkit-scrollbar-button:vertical:increment{
background-image: url('data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAcAAAAHCAMAAADzjKfhAAAAGXRFWHRTb2Z0d2FyZQBBZG9iZSBJbWFnZVJlYWR5ccllPAAAADlQTFRFEBIREhQSGx4VHSAWIiUYJCgZMTYePEIiPkQjRUwlSVAnXWYuanQziJY/nKxGp7hLsMFOwdRVyt5YRs1JvwAAACxJREFUCB0FwQsCQCAUALCl6CPi3f+wbQDGgXLJ3xzzTdwRUZFWPKD/J9CwARrSAPmqOpeUAAAAAElFTkSuQmCC');
background-position: 3px 4px}
/*
REM: Konnte ich nicht finden
::-webkit-scrollbar-button:horizontal:decrement:active {background-image: url(../images/Scrolling/black/horizontal-decrement-arrow-active.png); }
::-webkit-scrollbar-button:horizontal:increment:active {background-image: url(../images/Scrolling/black/horizontal-increment-arrow-active.png); }
::-webkit-scrollbar-button:vertical:decrement:active {background-image: url(../images/Scrolling/black/vertical-decrement-arrow-active.png); }
::-webkit-scrollbar-button:vertical:increment:active {background-image: url(../images/Scrolling/black/vertical-increment-arrow-active.png); }
*/
::-webkit-scrollbar-track{
-webkit-box-shadow: inset 0 0 6px rgba(0,0,0,0.3);
border-radius: 10px}
::-webkit-scrollbar-track-piece{background-color: #151716;}
::-webkit-scrollbar-thumb:vertical{
height: 50px;
background: -webkit-gradient(linear, left top, right top, color-stop(0%, #4d4d4d), color-stop(100%, #333333));
border: 1px solid #0d0d0d;
border-top: 1px solid #666666;
border-left: 1px solid #666666;
border-radius: 10px;
-webkit-box-shadow: inset 0 0 6px rgba(0,0,0,0.5)}
::-webkit-scrollbar-thumb:horizontal{
width: 50px;
background: -webkit-gradient(linear, left top, left bottom, color-stop(0%, #4d4d4d), color-stop(100%, #333333));
border: 1px solid #1f1f1f;
border-top: 1px solid #666666;
border-left: 1px solid #666666;
border-radius: 10px;
-webkit-box-shadow: inset 0 0 6px rgba(0,0,0,0.5)}
::-webkit-scrollbar-corner{background-color: #3D3D3D;}