Add CSS example with individual button styles (#57)

examples/css-style.jl

@@ -0,0 +1,54 @@
+# css-style.jl
+# Illustrate setting/changing individual button styles
+
+using Gtk4
+
+css = """
+window {
+    background-color: teal;
+}
+
+button:hover {
+    background-color: green;
+}
+
+.button2 {
+    background: orange;
+    color: purple;
+    font-style: italic;
+}
+
+.button3 {
+    background: grey;
+    color: red;
+    font-size: 18px;
+    font-style: normal;
+    font-weight: bold;
+}
+
+.button4 {
+    background: #FFCB05;
+    color: #00274C;
+}
+"""
+
+win = GtkWindow("Colorful Buttons")
+cssprov = GtkCssProvider(css)
+push!(Gtk4.display(win), cssprov) # applies CSS to all widgets
+
+b1 = GtkButton("Default style")
+b2 = GtkButton("Purple italiic")
+b3 = GtkButton("Bold red: Click me")
+add_css_class(b2, "button2") # goes with ".button2" in css
+add_css_class(b3, "button3")
+
+signal_connect(b3, "clicked") do widget
+    add_css_class(widget, "button4") # change color on click
+    widget.label = "Go Blue" # change label on click
+end
+
+box = GtkBox(:v)
+win[] = box
+push!(box, b1)
+push!(box, b2)
+push!(box, b3)