setTheme(theme) {
    if (this.colors.hasOwnProperty(theme)) {
      this.theme = this.colors[theme];
    } // TODO: this probably should notify those who are watching this theme
    // manager that the theme has changed so that colors can be changed
    // on the fly

  }
Sets the theme to the specified string if it exists. If it does not, this
does nothing.

@param {String} theme the name of the theme to switch to