There's yet another solution which might fit easily in everybody's project.
If you use a transparent border as a margin and a box-shadow with inset to set it's color, you'd get the result you are hoping for.
For instance:
::-webkit-scrollbar {
width: 10px;
}
::-webkit-scrollbar-track {
box-shadow: inset 0 0 10px 10px green;
border: solid 3px transparent;
}
::-webkit-scrollbar-thumb {
box-shadow: inset 0 0 10px 10px red;
border: solid 3px transparent;
}
This may be an ugly scrollbar, but it serves as an example to what I mean.
However, in Chrome, the transparent property won't work, so you would have to manually insert the color that would fit with the background, preferably, the background-color.