background-repeat should target div

[atotic]

without this div, core-icon icons repeat if size does not match

[ssorallen]

Styles should target classes with the key selector rather than the tag name where possible and appropriate. This could become:

core-icon.html

icon.style.backgroundImage = ...;
icon.classList.add('core-icon-icon');

core-icon.css

html /deep/ .core-icon-icon {
  background-position: center;
  background-repeat: none;
  background-size: 100%;
}

Or pull request #25 adds the necessary background-repeat on the icon's style object.