Seperate blog with single admin

I’m looking to create two topic areas for my site and in doing so would like the header and maybe a bit of color to change on the site depending on which post topic is being viewed, so it’s obvious which topic area is being viewed.

I originally thought of doing this by installing two seperate blogs on sub domains but this would mean users signing up for seperate accounts.

Is there anyway that I could keep the two blog topics seperate (different header etc) but keep one admin to control it all? I’m not sure whether the seperate sub-domain route is possible or if there is a way to change appearance of the site somehow depending on the topic, for example:

mysite.net/topic 1 – header and color scheme 1
mysite.net/topic 2 – header and color scheme 2

This is probably asking a lot but any guidance would be great.