fix(shell): change slider handle color to match trough

[isantop]

This better matches our GTK Theme as well as providing better contrast for the handle compared to the current version, which is the same as the background color.

Before:

After:

[jacobgkau]

Vastly increases visibility of the slider handle in the light theme. Both the light shell theme and the dark shell theme match the respective GTK4 themes after this change.

We'll want to update the screenshots in the theme switcher widget after this is merged.

I do want to point out that in the GTK3 theme, the handle is slightly lighter than the bar, which may help to differentiate it from the bar.

GTK3:

GTK4:

Was this design change within the GTK theme intentional?

[isantop]

Was this design change within the GTK theme intentional?

@jacobgkau The upstream GTK4 scale styling does not have a border on the filled trough, unlike the GTK3 theme:

The change to drop the border is intentional and IMO improves the cleanliness of the theme. The size different should suffice to visually distinguish the bar from the handle

[jacobgkau]

The change to drop the border is intentional and IMO improves the cleanliness of the theme. The size different should suffice to visually distinguish the bar from the handle

I saw the border, but I'm talking about the color. You're saying the GTK3 theme had no color difference between the handle and trough other than the border?

[isantop]

There is a (slight) color difference, but that's due to the way Adwaita set them up rather than a deliberate design choice. The new underlying Adwaita stopped modifying the trough color, so the two are now the same.