The Settings → Portal Appearance page can be reached by clicking the menu item.
Portal Appearance provides a way in which the user may choose the color in which Portal will open.
After a color is selected from the Colors drop-down list, click on Save Changes to apply the current setting for all users of the current tenant. If you have multiple tenants you can set a different color for each of them.
No restart is required in order the chosen color to be applied.