Theme Tools: do not load compat files in the dashboard.

[ryandhubbard]

Fixes #3679

Changes proposed in this Pull Request:

twentysixteen.css loads in the WordPress admin, but I can't find any page admin page where it is used. Added if statement to test for this.

Testing instructions:

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• Make sure your changes respect WordPress' coding standards.
• Did you make changes, or create a new .js file? If Gulp is installed on your testing environment, run gulp js:hint before to commit your changes. It will allow you to detect errors in Javascript files.
• Did you create a new action or filter? Add inline documentation to help others understand how to use the action or the filter.
• Create unit tests if you can. If you're not familiar with Unit Testing, you can check this tutorial.

[kraftbj]

Could you rebase this against master so the PR is only your changes?

[jeherve]

Closing in favor of #4865