I want to create a theme with my company's corporate colors.
I have an old Visio 2007 file which uses the colors.
I opened this in Visio 2013 file and saved as a Visio 2013 file.

When I go to Design tab, it indicates "No Theme".

Instructions I have seen on the web for creating a new theme seem to say that I should modify an existing theme and type in the RGB color codes. Is this correct?
When I click on a theme that I want to modify, the colors in my document are changed. I don't want to destroy any existing color information, as I will need the RGB codes etc.

And it doesn't seem like I can modify the "No Theme" theme.