Customize scrollbar to fit the theme colors

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Issue

Closes #4727: Sidebar's transient scrollbar doesn't fit in the Dark Theme

Approach

The 'webkit' API works on Chrome, but Firefox ignores it and uses it's own fancy scrollbar when 'webkit' is used (can't seem to change any properties). It's better than the current look, but still doesn't fit our theme, plus it makes the product look different on different browsers.

Firefox now supports the new 'scrollbar' API, so we can now tweak the scrollbar, but the API only provides limited attributes.

Don't wanna complicate things by using Javascript, so we'll use Firefox's limited attributes as the common denominator and just live with a plain, rectangle scroll bar BUT with better color to match the theme.

I think this color works for both lbry.tv and Odysee (both light and dark on each platform).

[neb-b]

This looks great! I think this should only happen on windows/linux though.

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@seanyesmunt, ah yes, forgot to mention that I didn't test on mac.
Is there an easy way to not include _scrollbar.scss globally when mac is detected?

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@seanyesmunt. Ah, I found it ... in ui\index.jsx

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@seanyesmunt Hm ... it didn't work. Was hoping to do something like the following, but import only works at the top level.
Is there an easy way to not include _scrollbar.scss globally when mac is detected?

// Import our app styles
// If a style is not necessary for the initial page load, it should be removed from `all.scss`
// and loaded dynamically in the component that consumes it
import 'scss/all.scss';

// Apply global scrollbar customization
const IS_MAC = process.platform === 'darwin';
if (!IS_MAC) {
  import 'scss/component/_scrollbar.scss';
}

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Update
Assuming I understood the change-request correctly, the second commit covers it. See commit message for details.

[neb-b]

Sorry I missed these messages. Was trying to get an app release out last week but have been having some build issues so this is going in too! Your solution looks great.