Computer Configuration > Administrative Templates > Google Chrome
Both
HKLM\Software\Policies\Google\Chrome
This policy allows admins to configure the color of Google Chrome's theme. The input string should be a valid hex color string matching the format "#RRGGBB". Setting the policy to a valid hex color causes a theme based on that color to be automatically generated and applied to the browser. Users won't be able to change the theme set by the policy. Leaving the policy unset lets users change their browser's theme as preferred. Example value: #FFFFFF