open file overall_header your theme
find:
after add:
<style type="text/css">
html, body{
scrollbar-face-color: #123456;
scrollbar-highlight-color: #123456;
scrollbar-shadow-color: #000000;
scrollbar-3dlight-color: #30303a;
scrollbar-arrow-color: #cc3333;
scrollbar-track-color: #242430;
scrollbar-darkshadow-color: #000000;
}
</style>