add InterVariable font

[terrifricker]

Add the following code from the Inter Font webpage so that InterVariable is used instead in browser that support variable fonts.

:root {
  font-family: Inter, sans-serif;
  font-feature-settings: 'liga' 1, 'calt' 1; /* fix for Chrome */
}
@supports (font-variation-settings: normal) {
  :root { font-family: InterVariable, sans-serif; }
}