code: function _replaceTheme( doc, theme ) {
      const HAS_THEME = /(theme-)[^"'\s]+/;
  
      if ( !theme ) {
          return doc;
      }
  
      const formClassAttr = doc.root().get( '/root/form' ).attr( 'class' );
      const formClassValue = formClassAttr.value();
  
      if ( HAS_THEME.test( formClassValue ) ) {
          formClassAttr.value( formClassValue.replace( HAS_THEME, `$1${theme}` ) );
      } else {
          formClassAttr.value( `${formClassValue} theme-${theme}` );
      }
  
      return doc;
  }
docs: Replaces the form-defined theme
  
  @param  {[type]} doc   libxmljs object
  @param  {string} theme theme
  @return {[type]}       libxmljs object