TextInput placeholder color is browser default rgb(117, 117, 117), not `black-300`

[dcapo]

The text color of the TextInput placeholder is lighter than it should be. The expected color is black-300 (i.e. rgb(103, 109, 115)), but the actual color is rgb(117, 117, 117). The latter is the browser default.

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Example
https://thumbtack.slack.com/files/U2N58F3NJ/F02MCNB26GJ/text_input__react____thumbprint.png