How to add backgroud image to bootstrap flatly? Hi Im not a theming hero,

Im an happy user of drupal8 and bootstap theme flatly.

I would like to have this sitebranding logo as a background image.



After 2 days of watching youtube tutorials about theming up bootstrap I can;t figure it out where to make the right modifications? 

please help
It's best practice to create a child theme on Bootstrap, the css modifications go into your child theme. That way you can update Bootstrap later on, otherwise your alterations would be overwritten by the update. More info on how to create a child theme can be found here