Allow font-family to be customised using CSS variables

[robcaldecottvelo]

Currently there is no way to override the font-family for some components (like button) without using:

• Custom CSS rules.
• The SASS $mdc-theme-font-family variable.

However, we can update the various theme colours using CSS variables which is simple and allows for easy custom theming (see the RMWC web site for an example.)

It would be great if the main theme font-family could also be overridden using a CSS variable in the same way as the various colours, e.g.:

:root {
  --mdc-theme-primary: #6200ee;
  --mdc-theme-secondary: #018786;
  --mdc-theme-font-family: "Custom Font Name"
}

[abhiomkar]

This is related to #4709

PRs welcome! Thanks.

[asyncliz]

Fixed in #5618