I think this is related by the fact that default document is using Bootstrap 3 and this framework has some specific CSS rules for printing. This is a limitation (issues) of Bootstrap 3.
Please see the discussion in
Setting !important is a way to make the CSS rules the most important, so it will override any other and so it will apply even during Printing.
As I explained in the issue linked above, you could use newer version of bootstap to avoid this issue.