Changing gtk.css files in /usr/share/themes/my-Yaru-dark doesn't change anything

I am trying to change the desktop icon font colour on ubuntu. I created a copy of the Yaru-dark theme named my-Yaru-dark. Then changed index.theme file as follows:

[X-GNOME-Metatheme]
Name=my-Yaru-dark
Type=X-GNOME-Metatheme
Comment=Ubuntu my-Yaru-dark theme
Encoding=UTF-8
GtkTheme=my-Yaru-dark
IconTheme=my-Yaru-dark
CursorTheme=my-Yaru-dark
CursorSize=24

Then I changed the gtk theme to my-Yaru-dark using

$ gsettings set org.gnome.desktop.interface gtk-theme my-Yaru-dark

I found gtk-2.0, gtk-3.0, gtk-3.20, gtk-4.0 directories in my-Yaru-dark. In each directory(except gtk-2.0), there was gtk.css and gtk-dark.css.

I tried looking for my gtk version. I ran $ gtk-launch --version. This gave output: 3.24.25. I assumed that I should edit gtk files in gtk-3.0 or gtk-3.20. In each gtk.css and gtk-dark.css file under both directories, after the import line, I added

.nautilus-desktop.nautilus-canvas-item {
    color: #000000;
    text-shadow: 1px 1px white;
}

Then I rebooted the system. I didn't observe any change. I am still using my-Yaru-dark theme but dektop font color is not changed.

I am using ubuntu 21.04. Thanks in advance.