[ntk] theme command implemented as ::itcl::extendedclass ::ntk::classes::theme mostly used from parts of ntkWidget (no direct use suggested)

Syntax:

Options:
    
    * '''-bd''': border width (default: '''0''')
    * '''-font''': font name (default: {})
    * '''-fontsize''': font size (default: {})
    * '''-text''': text for labels etc. (default: {})
    * '''-textcolor''': color for text (default: {})

MethodVariables:

    * '''textImage''' ''ntkWidgetImage'' for the text

Public commands:

Inheritance:

    * window

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