how to add speech bubbles background to the homepage

I would like to add speech bubbles backgrounds to pages I display on the homepage custom sidebar in one row (using plugin featured page widget (https://github.com/eduardozulian/a-featured-page-widget).
I found some html and css examples like this but now I wonder to which file shall I add html? To plugins or somewhere else? I´m using childtheme.

Thank you!