I don't quite have the time time to comb through am 11,000+ line CSS file, but I will save you some time by saying that you can style Constructs HTML elements by simply targeting their element followed by their type - rather than a class or id.
For example:
Textbox:
input[type="text"]{ /*Your CSS*/}
Button:
input[type="button"]{ /*Your CSS*/}
And so on.
If you require more granular control, you can style the elements by inlining the styles through editing the exports .html file
EDIT//
Having read your latest post - I would simply suggest editing the exported HTML file to include the material CSS as an external stylesheet and adding the classes and extra attributes outlined in the tutorial to the existing form elements (being careful not to change the ids, as I assume that's how Construct knows how to tie in event logic)