I've downloaded the theme at startbootstrap.com, uploaded the content onto the themes folder, I can see the theme on the backend although can't activate it. Is this already converted to use in october cms? or it needs to be completely re-edited in order to function with the october cms?

I don't think the version you need is at startbootstrap.com. The version you need has already been modified for octobercms. You can get it in the backend of your application in the settings part or check the public repo.