Creating a theme for existing bootstrap mvc page template

All the pages in my pure MVC website have Bootstrap's (3 column, footer, header) page template.  When I go to Design --> Page Template  and select this Bootstrap template, the top right corner has a button called "Themes".  When I click that I see a dropdown which allow me to select a theme. Currently, there is only Basic theme option in the dropdown and I want to use a custom  theme for this template which will be selectable from this dropdown. 
I am totally stuck as to how to go about to create a custom theme for an existing template. 
Request you to kindly provide me some instructions as to how to start.