As an administrator of your Office 365 tenant you have the ability to set a theme for everyone in your organization.
But when an user signs in for the first time the have the option to change their theme to something else like kittens
As a company this is something that you want to prevent and give all users the same user experience.
Because I saw this as a missing option I created a UserVoice request.
Some other people liked this but no official response from Microsoft until I saw a post from Wictor Wilén about an option custom tiles.
When I was looking there I saw an option to prevent users from overriding the custom theme
By selecting this your users can only select your theme or a high contrast version of your theme