Inline theme.scss to avoid deps issue downstream

[wwwillchen]

• Having a separate scss file means we need a sass_library target since sass_binary takes in one scss file as an input.
• Having a sass_library target requires a dependency on the "@angular/material" sass target, however we only have sass_external_binary but not sass_external_library so it's not trivial to have a sass_library with an npm dep like "@angular/material"

There's not much downside with inlining the theme, besides making the styles.scss file a bit longer.