You have to change your theme by installing custom third-party visual styles. Official Windows theme packs only include desktop background pictures, window colors, and sounds. Most custom themes are visual styles which can change the complete windows look - including the min/max/close buttons. In other words, with the right visual style your Windows 7 can look like Windows 8.
First, you'd have to find a visual style that appeals to you by browsing places like deviantART, Windows 7 Themes, or something similar. Just search these sites with your keywords and try to find something you like. If you can't find a suitable visual style, you can always create your own - depending on what you want to change it may take you anywhere from a couple of minutes to months to achieve.
Installing unsigned themes isn't as easy as installing an official theme pack and it's always a good idea to create a backup first but there are a lot of guides out there, like this one on Windows 7 Themes.