It’s easy to create a custom theme for the Office 365 browser apps (except Teams, which does its own thing). All you need is a corporate logo, some colors, and perhaps a sense of how colors go together. You can also block users from selecting from the default set of themes that come along with Office 365, an action that might just remove some light from some peoples’ lives.
https://office365itpros.com/2020/12/11/customizing-theme-microsoft-365-tenant/
