There are two major themes in Apple TV Light and Dark. For the most part the individual will select their theme and the app will respond to it. Your styles have can media queries to style the page differently depending on the theme.

You can also define a theme for a specific page. Doing so kicks in Apple's built in theme characteristics. Below is the sample markup for this and how the page differs when in dark and light modes.

<productTemplate theme="light">

Dark

Light