Enhancement: CSS variables for font weight

We added css variables to the design system which provide
font weights from 100 - 900:

--oc-font-weight-thin: 100
--oc-font-weight-extralight: 200
--oc-font-weight-light: 300
--oc-font-weight-normal: 400
--oc-font-weight-medium: 500
--oc-font-weight-semibold: 600
--oc-font-weight-bold: 700
--oc-font-weight-extrabold: 800
--oc-font-weight-black: 900

https://github.com/owncloud/web/pull/8504
