gh-142927: Add scrollbar support to Tachyon HTML dark mode

[Wulian233]

@hugovk

For compatibility, both approaches must be retained here.

::-webkit-scrollbar offers the widest compatibility, but requires fully customizing the scrollbar.
https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/Selectors/::-webkit-scrollbar#browser_compatibility

---

scrollbar-color is the newer standard, allowing us to style scrollbars without full customization. However, it is not supported by browsers from several years ago.

https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/Properties/scrollbar-color#browser_compatibility

• Issue: Improve Tachyon UX #142927

[hugovk]

These also apply to light mode, should they? And also * applies to every element.

But more importantly, this may override the user's OS scrollbar settings, which they may have customised for accessibility. Forcing them smaller and darker might mean it's hard for some to control or see.

[Wulian233]

I adjusted the scope and now only add it where needed

But more importantly, this may override the user's OS scrollbar settings, which they may have customised for accessibility. Forcing them smaller and darker might mean it's hard for some to control or see.

This has been reused according to the existing heatmap design standards, so they should not have much impact:)

cpython/Lib/profiling/sampling/_heatmap_assets/heatmap.css

Lines 815 to 827 in 1f36a51

	/* Scrollbar Styling */
	.line-content::-webkit-scrollbar {
	height: 6px;
	}
	.line-content::-webkit-scrollbar-thumb {
	background: var(--border);
	border-radius: 3px;
	}
	.line-content::-webkit-scrollbar-thumb:hover {
	background: var(--text-muted);
	}