If you have Simple Social Icons in more than one place on your site, make sure both have the exact same settings.
For instance, the Pretty Fabulous theme sets the background color automatically for the header left area, but the default background color for the simple social icons plugin is #ddd (dark gray).
If you then put a simple social icons widget into your sidebar, and the background is showing up gray, despite you having changed it to white, you’ll want to go check that other simple social icons widget in your header left area, and set that background to white.
Make sure you hit save.
If you continue to have issues with the simple social icons not displaying properly, navigate to Apperance > Widgets and scroll to the bottom of the page where it says “inactive widgets”. Be sure and delete any Simple Social Icons widgets from this area. Clear your cache. Then refresh your site.