Change theme scrollbar naming and set default dark scrollbars

This is a fix for #8384 that changes "Custom Scrollbars" to "Use Theme Scrollbars" (and makes the rest of the `customScrollbars` uses consistent with `themeScrollbars`)

Also moves the default dark scrollbars into the core UI theme so that turning off Theme Scrollbars with a dark theme will use Brackets' default dark scrollbar styling.