Install the extension or the startpage?
The extension is in the store so you shouldn't have any problems.
You don't "install" the startpage, and it's not a theme.
It's like a regular website - that's just in a HTML file - like the ones you often see online (sometimes a URL ends with .html, that's what this is). The difference is that isn't not online and you just open the file.
To get started using it, download the file and extract it to a desired location. I'd recommend somewhere out-of-the-way, because you will need to only access it once during set up. Then, install the extension and where it asks for a URL or file, put the location of the .html file in the download. You'll need to direct exactly to the html, and not just the folder its in. You will also have to add "file://" to it, as it shows on the page.
Hope that helps