In your custom styles.less, add:

.scrollbars-visible-always {
.tool-panel ::-webkit-scrollbar {
min-width: 6px !important;
width: 6px !important;
min-height: 6px !important;
height: 6px !important;
}
/deep/ ::-webkit-scrollbar {
min-width: 6px !important;
width: 6px !important;
min-height: 6px !important;
height: 6px !important;

&-thumb {
border: none;
border-radius: 0px !important;
background-color: #69c !important;
}
&-thumb:active {
background-color: #69c !important;
}
&-track {
background-color: #000 !important;
}
}
}