Paste the entire contents of that file into Design → Custom CSS → Save.

It styles the native Newsletter Block to match the site — transparent background, Helvetica, small caps labels, underline-only input field, dark ink button with paper-tone text. The newsletter block heading and description text are set in Squarespace's block editor itself, so set those to:

• Title: Get Email Updates

• Description: Sign up to our monthly newsletter to receive updates on Last Night at Lounge Ax and other projects in the works.