There are 4 different theme colors in Microsoft office 2016 and 2019. The office theme by default is "Colorful" theme, but you can change to any of the other 3 themes (Dark Gray, Black, White). Please follow the steps below for details:
Step 1: Click the "File" tab in the ribbon;
Step 2: Click "Options" from the left menu;
Step 3: Click "General" in the "Outlook Options" window;
Step 4: In the "Personalize your copy of Microsoft Office" section, select a color from the drop-down list as the office color;
Step 5: Click "OK" at the bottom to close the window.