inject(contents) {
    var links = [];
    var themes = this._themes;
    var theme;

    for (var name in themes) {
      if (themes.hasOwnProperty(name) && (name === this._current || name === "default")) {
        theme = themes[name];

        if (theme.rules && Object.keys(theme.rules).length > 0 || theme.url && links.indexOf(theme.url) === -1) {
          this.add(name, contents);
        }

        this._injected.push(name);
      }
    }

    if (this._current != "default") {
      contents.addClass(this._current);
    }
  }
Inject all themes into contents
@param {Contents} contents