[Accessibility] Use REM instead of pixels for vivid CSS

[acelletti]

We are currently seeing that as we change the font size of our browser, all of the vivid components are not scaling with the change in size.

This creates a weird effect where our website is mostly responsive, with the vivid components sticking out as too big / too small.

To reproduce simply change your browser font size and visit https://vivid.vonage.com/ to see that the page stays the same.

[acelletti]

[yinonov]

It's been resolved

https://vivid.deno.dev/designs/design-tokens#typography