Make it match your own phone
This page opens in a light Messages-app style: a white background, a very pale blue header, soft grey-blue bubbles for the other person and light blue bubbles for you, all with dark text. Google Messages colors can change with each phone’s theme, so yours may look a little different.
To match it, open the style menu above the chat, choose Custom colors and set the background, header, left bubble and right bubble. The preview, the downloaded image and the video clips all use your colors.