Dark themes, if you aren't already aware, introduce power savings for OLED displays as they shut off individual pixels when showing the color black. They're also just a different look and it's pretty cool.
Overall, Files by Google executes the look okay, though there are some rough spots. For example, the hamburger menu icon at the top-left corner isn't colored correctly for visibility and the file clearing animation could use a better color scheme. But it's a pretty darn decent take as it is.