Add custom properties support to modals

[sebnitu]

What changed?

This PR primarily applies custom properties support to the modal component. This is done in a similar way to how popover custom properties are applied in that they are provided fallback values and set the custom property names to override but do not output the custom properties directly.

Additional Changes

• modal.active now returns null instead of undefined if there are no active modals.
• Refactors focus-ring mixin to allow passing the opacity value of the color.
• Apply focus-ring to modal and remove outline variables.
• Added focus-ring and full scale variables.
• Apply new BEM naming function to modal component and remove prefix variables.