NOTE: Previous what to set the theme globally was through editing the overrides.css file. For those that have done that, you should navigate to the location the portal is installed. That will typically be in c:\inetpub\Live Maps Unity Portal\Conent. Find the overrides.css file and rename it something else.
As of V8.7, in order to set a global theme, open up IIS Manager on the server(s) that the Live Maps Portal is installed.
Navigate to the Live Maps Portal website. This can be a seperate website or a child application under a website.
Double click on Application Settings.
Add the following
Values can be the following;
dark - for the Dark theme
blue - for the Blue theme