Question on switching between themes

I am completely new to WordPress but have designed a blog for myself shown here:
My Blog

I am trying to develop a new theme for a new website for a friend. I have mocked up the website in all html, along with the CSS file to make it work. This page is parked here:
My new website

I was hoping I could convert this html file to a WP compatible index page and be able to switch between my normal blog shown above, and this new website.

I have put all the files for a blank theme into a folder, including my new CSS file, and have uploaded it to the same WP site in the theme folder.

When I activate this new theme, the content from my original blog is shown My question is, once I convert my html file to a WP compatible index page, will I be able to switch between my original blog and this new website? I don’t understand why my original content is showing with the new template.

I have done some searching, but haven’t found an answer to my question.

Lawrence