title: Python-docx unable to use existing document - no style with name 'Title'
body: <p>I'm trying to make a program that will create a document based on a text file and so far, it's worked fine. I decided that, to make it easier to use images and other things not supported/difficult to use effectively in Python-docx. Upon using exactly the same code but instead using Doc = document() I use Doc = document("template.docx"). After modifying, the file is saved to a different docx file. I get these errors when trying to use the template. There are no errors when creating a new document.</p>
  
  <p>Traceback (most recent call last):</p>
  
  <pre><code>  File "C:\Users\bgrif\Desktop\QPA.py", line 45, in &lt;module&gt;
      Doc.add_heading("QuizPax 28/02/2019",0)
    File "C:\Users\bgrif\AppData\Local\Programs\Python\Python37-32\lib\site-packages\docx\document.py", line 39, in add_heading
      return self.add_paragraph(text, style)
    File "C:\Users\bgrif\AppData\Local\Programs\Python\Python37-32\lib\site-packages\docx\document.py", line 56, in add_paragraph
      return self._body.add_paragraph(text, style)
    File "C:\Users\bgrif\AppData\Local\Programs\Python\Python37-32\lib\site-packages\docx\blkcntnr.py", line 39, in add_paragraph
      paragraph.style = style
    File "C:\Users\bgrif\AppData\Local\Programs\Python\Python37-32\lib\site-packages\docx\text\paragraph.py", line 111, in style
      style_or_name, WD_STYLE_TYPE.PARAGRAPH
    File "C:\Users\bgrif\AppData\Local\Programs\Python\Python37-32\lib\site-packages\docx\parts\document.py", line 78, in get_style_id
      return self.styles.get_style_id(style_or_name, style_type)
    File "C:\Users\bgrif\AppData\Local\Programs\Python\Python37-32\lib\site-packages\docx\styles\styles.py", line 109, in get_style_id
      return self._get_style_id_from_name(style_or_name, style_type)
    File "C:\Users\bgrif\AppData\Local\Programs\Python\Python37-32\lib\site-packages\docx\styles\styles.py", line 139, in _get_style_id_from_name
      return self._get_style_id_from_style(self[style_name], style_type)
    File "C:\Users\bgrif\AppData\Local\Programs\Python\Python37-32\lib\site-packages\docx\styles\styles.py", line 53, in __getitem__
      raise KeyError("no style with name '%s'" % key)
  KeyError: "no style with name 'Title'"
  </code></pre>
  
  <p>Does anyone know how to fix this? Thanks in advance.</p>