code: void appendProperty( @Nonnull String name, @Nonnull Expression value ) {
          if( output == null ) {
              throw new LessException( "Properties must be inside selector blocks, they cannot be in the root." );
          }
          insets();
          name = SelectorUtils.replacePlaceHolder( this, name, value );
          output.append( name ).append( ':' );
          space();
          value.appendTo( this );
          if( state.importantCount > 0 || value.isImportant() ) {
              output.append( " !important" );
          }
          semicolon();
          newline();
      }
docs: Append a property to the output like: name: value;
  
  @param name
  the name
  @param value
  the value
  @throws LessException
  if write properties in the root