fix(multiple): use inline m2 token definitions

[andrewseguin]

Defines M2 theme tokens (CSS vars) inline instead of creating a new selector block

// Theme file:
html {
  @include mat.button-theme($theme);
  @include mat.card-theme($theme);
  @include mat.checkbox-theme($theme);
}

Before, this would have split each component into its own html block:

html {
  // button token CSS variable definitions
}

html {
  // card token CSS variable definitions
}

html {
  // checkbox token CSS variable definitions
}

With this change, these will be grouped together:

html {
  // button token CSS variable definitions
  // card token CSS variable definitions
  // checkbox token CSS variable definitions
}

For some users, this will surface a Sass warning about mixed-decls. This was previously being avoided because the CSS created the variables in subsequent selector blocks, but are now inline.