Check Icon Font Awesome. You can also add larger icon classes to the parent to further control the sizing. Icons can be used in your survey to create visual help and add meaning to elements.
A complete list of font awesome icon class names along with their corresponding unicode values. Example of check at 6x example of check at 5x example of check at 4x example of check at 3x example of check at 2x example of check. The amazing font awesome icon set.
The last point is to make sure the content code used in css is correct.
Available source files and icon fonts for both personal and commercial use. The png format is widely supported and works best with presentations and web design. I know this is an old question, but hopefully my answer can help someone who wants a thinner circle when stacking two font awesome icons. Replace font awesome with modern line icons with a single line of code.