gtk-rs change widget appearance at runtime.

#!/usr/bin/env -S cargo +nightly -Zscript
---
[dependencies]
gtk = { package = "gtk4", version = "0.9.3", features = ["v4_12"] }
---

use gtk::{glib, prelude::*};

const STYLESHEET: &str = r#"
.green {
    color: green;
}
.red {
    color: red;
}
"#;

fn main() -> glib::ExitCode {
    let app = gtk::Application::builder()
        .application_id("org.example.HelloWorld")
        .build();

    app.connect_activate(|app| {
        let window = gtk::ApplicationWindow::builder()
            .application(app)
            .title("Hello, World!")
            .build();

        // Stylesheets are usually bundled with application resources
        // and automatically loaded
        let css_provider = gtk::CssProvider::new();
        css_provider.load_from_string(STYLESHEET);
        gtk::style_context_add_provider_for_display(
            &RootExt::display(&window),
            &css_provider,
            0
        );

        let box_ = gtk::Box::new(gtk::Orientation::Vertical, 6);

        let label = gtk::Label::builder()
            .label("Hello, World")
            .css_classes(["green"].as_slice())
            .build();
        box_.append(&label);


        let button = gtk::Button::builder()
            .label("Toggle Color")
            .build();
        box_.append(&button);

        button.connect_clicked(glib::clone!(#[weak] label, move |_| {
            if label.has_css_class("red") {
                label.add_css_class("green");
                label.remove_css_class("red");
            } else {
                label.add_css_class("red");
                label.remove_css_class("green");
            }
        }));

        window.set_child(Some(&box_));
        window.present();
    });

    app.run()
}