Rename classes, more button options via StyleProvider (#8464)

Moves global classnames to e.g. `egui::class::ROOT`.

Widget specific classes live on the widgets struct. 

Adds some more classes for our buttons styling options (`CLASS_SMALL`
etc), so that custom `StyleProvider`s can choose to implement them
differently or ignore them altogether.


Also removes some styleproviders that were partly or completely unused
(e.g. basestyle / labelstyle). We can add them back once we actually
implement styling for those.

---------

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

examples/styling_engine/src/main.rs

@@ -4,9 +4,10 @@
 //! based on the _classes_ set on it, and that can be edited live.
 
 use eframe::egui::{
-    self, CentralPanel, Color32, Frame, Panel,
+    self, CentralPanel, Color32, Frame, Panel, TextStyle,
+    class::HasClasses as _,
     theme::StyleProvider,
-    widget_style::{BaseStyle, ButtonStyle, HasClasses as _, StyleArgs, WidgetState},
+    widget_style::{ButtonStyle, StyleArgs, TextVisuals, WidgetState},
 };
 
 /// Buttons with this class are styled as a destructive action.
@@ -42,14 +43,11 @@ impl StyleProvider<ButtonStyle> for MyTheme {
         let StyleArgs {
             classes,
             state,
-            ctx,
+            style,
             ..
         } = args;
 
-        // Start from the style egui computed for a generic widget, so we inherit e.g. the font:
-        let base: BaseStyle = ctx.get_widget_style(args);
-
-        let fill = if classes.has(DANGER) {
+        let fill = if classes.has_class(DANGER) {
             self.danger
         } else {
             self.normal
@@ -63,11 +61,13 @@ impl StyleProvider<ButtonStyle> for MyTheme {
         };
 
         ButtonStyle {
+            min_size: egui::vec2(0.0, style.spacing.interact_size.y),
             frame: Frame::new()
                 .fill(fill)
                 .corner_radius(self.corner_radius)
                 .inner_margin(8),
-            text_style: base.text,
+            // Resolve the font from the style, so we follow the user's font sizes:
+            text_style: TextVisuals::new(style, TextStyle::Button, Color32::WHITE),
         }
     }
 }