I recently completed the quickstart tutorial and added the dream theme as a submodule of my project. I was wondering if/how I can use my own html code for the homepage, and redirect the theme’s index.html code for the homepage to another page.
It’s normally a matter of copying the template for the homepage to the template that generates the page you want to style, and adjust as needed, such as removing code that only makes sense of the home page.
Please check out the entire section in the docs concerning templates, especially lookup order. 
Exactly what @maiki said.
The best way to do this is to put your own index.html (homepage) in your website’s layout directory. This would override the theme’s layout/index.html.
For more information on this topic, check out the documentation