I can't remember the exact name for the themes I used, but if your go into the Linux Mint theming section and search "Windows" you will get several results.

I don't know if there is a Windows 7 theme specifically, you would have to look for that yourself. I also did little things like allign and resize their desktop icons the same way their Windows desktop looked. I changed the default folder colors to a tan-ish color to look similar to the Windows folder colors. My mom could tell it looked different, but it was close enough.

Making their app icons look the same and be in the same rough location as their Windows machine is probably the most important. My Mom loves the Spotify desktop app, so I made sure to install it from the software center and pin the icon into the taskbar right where she was used to seeing it.

Make sure their browser home page is set the same too, and any bookmarks they have.

Also, guide them through the new install. Have them click through all the typical tasks they do. I had my mom sit with me and showed her how Spotify opened up and looked exactly the same as it did on her Windows install. We played some music and I showed her how to adjust the little volume knob in the Mint toolbar. I had her print some documents, browse the web, look at pictures and videos she had saved on her drive, stuff like that.

That will make them feel much more comfortable with the change. There is a balance between trying to get everything to look identical, and helping your parents become comfortable with something new.