I wonder how to stop loading/disable grid-flexbox.css. I would like to use Bootstrap's media query breakpoint units in "px" instead of "em" used by grid-flexbox.css. I find it confused when I have both loading on the same site.
Besides, I wonder if this default grid-flexbox breakpoint originally adapting Bootstrap's breakpoint ? It seems Bootstrap changed to "em" once but now it's changed back again to "px" in v4. Perhaps it needs an update if so.
The easiest way to remove grid-flexbox.css is to tell your theme to include a file with that same name, and then not include the file. So in your theme's themename.info file, simply add the line:
If, however, you wanted to replace the bootstrap4 gridsystem with one of your own (with px instead of em, or however you like...) you can also include a file with that exact same name in your theme, and your copy will replace the one added by Backdrop core.