[css-scrollbars-1] Remove light and dark keywords of scrollbar-color in favor of color-scheme

[fantasai]

Given that

• The (more recently added) color-scheme property affects all UA-generated UI colors, including scrollbar colors.
• Firefox doesn't implement light and dark yet anyway.

I'm thinking we should remove these two keywords and just let color-scheme manage the colors for scrollbar-color: auto as needed.

[css-meeting-bot]

The CSS Working Group just discussed scrollbar-color: light/dark, and agreed to the following:

• RESOLVED: drop the light/dark keywords from scrollbar-color

The full IRC log of that discussion

<fantasai> topic: scrollbar-color: light/dark
<fantasai> github: https://github.com//issues/6438
<TabAtkins> fantasai: When we first added sc4rollbar-color we didn't have color-scheme
<TabAtkins> fantasai: So we had keywords for light/dark so authors could request those if they just wanted to match their theme
<TabAtkins> fantasai: Since then we've added color-scheme which does this on a broader scale, and in particular shoudl automatically change the scrollbar colors
<emilio> +1
<TabAtkins> fantasai: And nobody's implemented light/dark anyway
<lea> +1
<bmathwig> +1
<florian> +1
<TabAtkins> fantasai: So proposal is to just drop these values
<emilio> q+
<TabAtkins> +1
<tantek> +100
<astearns> ack emilio
<TabAtkins> emilio: We don't implement color-scheme, but we do have darkening of scrollbars vs the background color
<TabAtkins> emilio: So perhaps shouldn't specify it should follow the color scheme
<TabAtkins> emilio: We currently get away with auto-darkening scrollbars on pages that don't use color-scheme
<tantek> this is a good point, there may be a compat need to keep 'dark'
<TabAtkins> florian: default value of color-scheme is "auto" anyway, we can make sure there's some flexibility there
<TabAtkins> tantek, there's zero implementation of 'dark', so by definition no compat need
<TabAtkins> astearns: Objections?
<tantek> TabAtkins: huh, ok then I misunderstood emilio
<TabAtkins> RESOLVED: drop the light/dark keywords from scrollbar-color
<emilio> tantek: firefox does darken scrollbars of scrollers with dark backgrounds
<TabAtkins> emilio was concerned about the auto value *requiring* the scrollbar to go light/dark depending on 'color-scheme'
<tantek> emilio, got it. so this is allowing that flexibility in 'auto'
<emilio> tantek: (assuming scrollbar-color: auto ofc)
<emilio> tantek: right