How to use CSS customization directly from Vala? Not like in this example with file. Let's say I want the button to turn red by clicking on it, without using an external css file, as this action is too simple to create a css file with a single field.
I mean smth like this:
label.set_styleSheet("font-size: 17px;")
You still have to create a CssProvider, like in the code you linked to:
var screen = this.get_screen ();
var css_provider = new Gtk.CssProvider();
You can call load_from_data () instead of load_from_path () to load it from a string in memory instead of a file:
https://valadoc.org/gtk+-3.0/Gtk.CssProvider.load_from_data.html
css_provider.load_from_data(".my_class { font-size: 17px; }");
Gtk.StyleContext.add_provider_for_screen(screen, css_provider, Gtk.STYLE_PROVIDER_PRIORITY_USER);
When the CSS provider has loaded the custom styles to be used, you can manipulate every Gtk+ widget with get_style_context ().
The style context has methods to add, remove and query a class, etc.
https://valadoc.org/gtk+-3.0/Gtk.StyleContext.html
label.get_style_context().add_class("my_class");
Since you only have to setup the style provider once, I don't think it is too much overhead.
For anyone who is reading this I have posted both examples with and without file to Git https://gitlab.com/gavr123456789/vala-css-examples/tree/master