How to recolor scrollbar thumb? I have partial solution, but it's not complete

[XCanG]

I'm using theme Dracula and this theme is overall ok, however the scrollbar thumb is #000 with a dark background which make it hard to see. This is how it look:

Now that I have a lot of tabs and thumb become small I just can't see it, so I decided to recolor it, however in my attempts I failed.

A typical CSS would involve something like scrollbar-color, I tried to apply that to various places, but it didn't work. It work in the browser and on websites, like github, but didn't work in side bar. I do already have some styles which work, but this one doesn't.

My next attempt was to use browser devtools (Ctrl+Shift+J) and there I find that it is not typical blocks:

With some testing within devtools I find out that styles like:

thumb {
    appearance: none !important;
    background: #8ef;
}

would work, so I added the style for #tabbar > #normal-tabs-container > scrollbar > slider > thumb, but it didn't work, nor for just thumb selector.

So I'm a bit stumbled and cannot proceed any further.

[irvinm]

Check this out: https://github.com/piroor/treestyletab/wiki/Code-snippets-for-custom-style-rules#custom-scrollbar-colors-2911

[XCanG]

This one works, but it strange, when I tried it myself I applied it to various selectors, like :root, #tabbar, etc. but it doesn't affect it. This selector #normal-tabs-container does.