Sorry for the long response. While I was creating an example, I got a notice that in the docs example
'{{#pptxStyle textColor='0000FF'}}Simple text{{/pptxStyle}}',
the color is specified without the '#'. However, I had written it as
'{{#pptxStyle textColor='#0000FF'}}Simple text{{/pptxStyle}}'.
So its all my mistake.
I hope this post helps others as inattentive as I am.