In the past, Dynamics came with one default theme, and creating a new theme was a bit of a pain–especially if you were trying to just differentiate between a sandbox and production environment. Microsoft now has three out-of-the-box themes for you to choose from and copy/customize.
To check out themes…
…from the unified user interface (any of the “hubs”):
Select the gear icon in the upper right and select “Advanced Settings”.
…from the classic/legacy web view:
From here, you’ll see three themes that you can preview by selecting the theme, then “Preview” from the ribbon menu: