I don't think I like the way that pop-os rounds all the corners. I have been trying to find the best way to edit the theme and the below seems to work.

This seems to do it, not sure how successfully. Found on r/pop_os, posted by user Sorel_CH.

I have been trying for hours to get rid of the rounded window corners in gnome/gtk, any ideas?

decoration, window, window.background, window.titlebar, * {
border-radius: 0px;
}

The css files for the themes are located here -

/usr/share/themes/...

Issues

Don't think the css change is persistent across state changes (i.e. restarts etc).