Hi,
The system indicator on the Company Information helps you in this case and you can specify the System Indicator Style as well, based on which it'll be highlighted on the every page. If you are talking about different themes for different companies, I feel it will be more confusing as most of the users have to remember each color or theme code (especially when they are new or you may have maintain another legend for each color code).
The system indicator would give a clear idea as to which company you are in.
Best Regards,
MMV