Theme parser error: Expected a valid color

Question

I want to change the color of the "checkmark" icon to blue, but the color does not change. Instead, I get following error:

(main.py:23000): Gtk-WARNING **: 15:13:17.402: Theme parser error: style.css:25:58-59: Expected a valid color.

My CSS code looks like this:

.checkmark {
    -gtk-icon-style: symbolic;
    -gtk-icon-source: -gtk-recolor(url("checkmark"), blue);
}

This is how I implement the icon (using Python):

checkmark = Gtk.Image.new_from_gicon(Gio.Icon.new_for_string("checkmark"))
checkmark.add_css_class("checkmark")
#I later append it to the window

How can I recolor the checkmark icon in GTK4?