feat(MaterialApp): theme scrollbars

[Florian-Schoenherr]

squash (next time I will lint before commiting)
Closes: #56

[vercel]

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[Florian-Schoenherr]

It doesn't work on vercel. And I don't know why.

[TheComputerM]

The vercel site is very buggy, I cannot scroll and the scrollbars look weird.

[Florian-Schoenherr]

Can't convert this to draft, and I guess I still need some time.

Found something: #root needs to be height: 100%.
This should just be mentioned first on the MaterialApp docs. If you want, I can try some different setups (using two MaterialApps, etc).
z-index on the aside is the culprit. Scrollbars don't have z-index (on chrome), so best idea for now:
put margin-left: <scrollbar-width> on the side-menu.
I will push a smaller scrollbar later.

Right now we have another problem:

It's not updating with the new scrollbar, let me see what I can do.
Update:
cferdinandi/gumshoe#114 okay, gumshoe apparently only works on the "main scrollbar".
I mean, something like gumshoe which has the option to also specify the container on which scrolls happen would be nice.
We could leave this open until I get the time to find an alternative or anybody has a better idea.