If you want your Windows 7 themes to change some of your icons, you first need to check the option “Allow themes to change desktop icons”
Steps to apply desktop icons in Windows 7
1. Step First of all, click on the Start >> Control Panel
2. Step Under Appearance and Personalization, click on the ‘Change the theme’.
3. Step Click on the ‘Change desktop icons’ on the left side of the window.
4. Step Select ‘Allow themes to change desktop icons‘ and click Apply. And then OK.
5. Step Now if you change theme, then the theme icons are automatically applied. Let’s try once. Take a look at your icons before changing theme. Then open the Personalization by clicking Start >> Control Panel >> Change the theme.
6. Step Apply a new Windows 7 theme that includes desktop icons. To use the theme click on the theme. If the theme does in fact have some icons, the icons will change. This can be the Computer icon, the Network icon, the Recycle bin icon, the User icon, the Control Panel icon and various other icons.
Not all Windows 7 themes include different icons but if they have new icons it will change automatically after you apply a theme. It is important to Allow themes to change desktop icons as specified in the step – 4.