Remove the borders, and make each thumbnail have a background color that matches other elements like the Dash. Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1593>
While workspace switcher is an OSD, workspace pager is an overview panel. So it makes more sense to separate them into different stylesheets. https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/938