I will try to look on this

One problem tho as I always say I'm a complete newbie. this is under gnome-shell/ right?

Modifying border-radius and the ligthen function :&active should solve this.

.notification-banner {
  min-height: $notification_banner_height;
  width: $notification_banner_width;
  box-shadow: 0 2px 4px 2px $shadow_color;
  margin: $base_margin;
  border-radius: $modal_radius;
  background-color: $bg_color; // Yaru change: use bg_color for better contrast with buttons

  @if $contrast == 'high' {
    @include draw_hc_inset();
  }

  &:hover {
    // Yaru change: ↑↑↑
    background-color: if($variant =='light', darken($bg_color, 2%), lighten($bg_color, 2%));
  }

  &:active {
    // Yaru change: ↑↑↑
    background-color: if($variant =='light', darken($bg_color, 4%), lighten($bg_color, 4%));
  }

  // Yaru: make notifications more visible on different headerbars:
  &,
  &:hover,
  &:focus,
  &:active {
    box-shadow: 0 0 4px 2px rgba(0, 0, 0, 0.1);
    border: 1px solid $borders_color;
  }
}

build from modified source:

the ninja installation work I looked at the changes in /usr/share/gnome-shell and confirmed that the 60px border radius I tested is in there but the popup notification is still in the default radius

manually edited gnome-shell.css:

I also tried changing gnome-shell.css in /usr/share/themes/Yaru/Yaru-dark ... log out login ... no luck.

I'm not familiar into css theming in Yaru at all so maybe there are necessary files to be edited too? 🤷🏽

@Feichtmeier
Why is the gnome-shell-theme.gresource file not being overwritten? the ninja installation doesn't even install it.
looked upon gnome-shell/theme/Yaru still no gnome-shell-theme.gresource installed

Did I miss something?
I tried to extract the gnome-shell.css and other variants in that gresource file and looked at the .notification-banner entry but it is still the default 6 px.

I followed the build and install steps

$ meson _build
$ ninja -C _build
$ ninja -C _build install

even tried to uninstall yaru-theme-gnome-shell then repeated the above commands, looked upon gnome-shell/theme/Yaru still no gnome-shell-theme.gresource installed

Did I miss something?

@Feichtmeier
Why is the gnome-shell-theme.gresource file not being overwritten? the ninja installation doesn't even install it.
looked upon gnome-shell/theme/Yaru still no gnome-shell-theme.gresource installed

Did I miss something?
I tried to extract the gnome-shell.css and other variants in that gresource file and looked at the .notification-banner entry but it is still the default 6 px.

I followed the build and install steps

$ meson _build
$ ninja -C _build
$ ninja -C _build install

even tried to uninstall yaru-theme-gnome-shell then repeated the above commands, looked upon gnome-shell/theme/Yaru still no gnome-shell-theme.gresource installed

Did I miss something?

I'm a little out of the loop but the last time I edited the notifications all I needed to change was scss code
Forwarding this to @3v1n0 and @Muqtxdir

thanks!