changeActive(themeName) {
    var _this = this;
    var newActiveTheme;

    if (!themeName) return Promise.reject(new Error('theme - changeActive() - themeName is required'));

    return _this.list()
      .then((themes) => {
        return new Promise((resolve, reject) => {
          newActiveTheme = _.findWhere(themes, {
            name: themeName
          });

          if (!newActiveTheme) return reject(new Error(`theme - changeActive() - ${themeName} is not a valid theme`));

          themes = themes.map(theme => {
            theme.active = false;
            return theme;
          });

          newActiveTheme.active = true;

          return resolve(themes);
        });
      })
      .then(writeThemesFile)
      .then(readThemesFile)
      .then(() => {
        return new Promise((resolve) => {
          return resolve(newActiveTheme);
        });
      });
  }
Change active theme
@param {string} themeName - Name of the theme to change to