Use design tokens for the 2FA specific (admin) styling

[sergei-maertens]

The package (from upstream) includes a 2fa.css file which is used to style the wizard - amongst others it includes a background color for the .btn class.

This does not work well together with our custom admin styling overrides, causing mismatched colors that are not easily override-able.

The best solution would be if 2FA uses design tokens for this, which can be included easily by downstream projects.

Example:

Project CSS:

:root {
  --2fa-btn-background: red;
  --2fa-btn-color: white;
}

2fa.css:

.btn {
    background: var(--2fa-btn-background, #79aec8);
    color: var(--2fa-btn-color, #fff);
    /* other rules... */
}