scrollable tracks should not inherit css from ancestors

[yairEO]

it could be that multiple Scrollable components are nested, and so, their CSS styles affect each other because of how the tracks selectors are structured:

.scrollbar.vertically-scrollable .vertical-scrollbar-track

which means nested components with the class .vertical-scrollbar-track will be affected.

This fix assumes the tracks are always direct children of a .scrollbar parent

see src/components/Scrollable/components/_tracks.scss

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (b4e2c3a) 96.01% compared to head (0ebe9ea) 96.01%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #119 +/- ## ======================================= Coverage 96.01% 96.01% ======================================= Files 61 61 Lines 853 853 Branches 29 29 ======================================= Hits 819 819 Misses 33 33 Partials 1 1 ```

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