Proposal: remove background-color from .mat-drawer-container

#### Bug, feature request, or proposal:
Proposal: remove `background-color` from `.mat-drawer-container`

#### What is the expected behavior?
The common background that I've set in my app appears on a page with a drawer

#### What is the current behavior?
Putting a drawer on a page requires putting the page contents inside `md-drawer-container` which comes with `background-color` set that overrides any app background that's been set.

#### What are the steps to reproduce?
Add a drawer to a page with a non-standard background

#### What is the use-case or motivation for changing an existing behavior?
Overlays for drawers and sliders are typically supposed to block the majority of the UI. IMHO material shouldn't be providing a background-color for this majority.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
2.0.0-beta.10

#### Is there anything else we should know?

Commencez par rechercher `.mat-drawer-container` dans les styles de Angular Material ainsi que le point d’entrée du composant `md-drawer-container`, puis reproduisez le problème avec un arrière-plan d’application non standard et un drawer. Le travail est terminé lorsque l’arrière-plan de l’application reste visible derrière le contenu de la page du drawer sans perturber le comportement du drawer, et que les tests pertinents ont été mis à jour ou ajoutés si cette zone est couverte.

Rédigé par le modèle d'indexation à partir du texte de l'issue.