Update _base.css

[mattbrundage]

Vendor-prefixed ::selection is no longer needed, per our browser support matrix.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

Pull requests should be thought of as a conversation. There will be some back and forth when trying to get code merged into this or any other project. With all but the simplest changes you can and should expect that the maintainers of the project will request changes to your code. Please be aware of that and check in after you open your PR in order to get your code merged in cleanly.

Thanks!