docs(card): share icon button in dark mode doesn't match other icons in demo

[EdricChan03]

Actual Behavior:

• What is the issue? The share button at mdCard for dark theme is grey. (Extra-large image)
• What is the expected behavior? The share button should also be white.

CodePen (or steps to reproduce the issue): *

• CodePen Demo which shows your issue: https://material.angularjs.org/HEAD/demo/card
• Details:

Angular Versions:

• Angular Version: 1.5.8
• Angular Material Version: HEAD

Additional Information:

• Browser Type: Chrome
• Browser Version: 55.0.2883.35 beta (64-bit)
• OS: macOS Sierra 10.12.1
• Stack Traces:

---

Shortcut to create a new CodePen Demo.
Note: * indicates required information. Without this information, your issue may be auto-closed.

Do not modify the titles or questions. Simply add your responses to the ends of the questions.
Add more lines if needed.