Apply borders to Figma components for high contrast theme

[andrew-ronaldson]

In most High contrast themes borders are used to show clear separation between objects on the screen. In our default theme, borders are not used on all components so in order to enable a true high contrast theme we need to support transparent borders on all borderless components.

Figma has a built in theme switcher that can instantly swap styles to different themes and modes. Adding a new transparent border token would help us take advantage of this functionality rather than maintaining extra theme files.

[lboehling]

Update --

Tina has been evaluating when/where to add borders in this figma file.

I've explored figma variables for applying the border in this file.

• Exploring new high contrast border color variable that is set to transparent in L/D themes, and aliases to border--color--default in HCL/HCD.
• Exploring new boolean border variable that is set to False in L/D themes and true in HCL/HCD.

*I think we may need to use a combination of the 2 proposed tokens to enable HC border toggles in figma (still evaluating though). POC will test to see if these proposed tokens can work in core as well.

Currently working on documenting a list of components that need a net-new border, and document standards around border styling updates across components.