Follow the System Theme in egui (#4860)

* Some initial progress towards #4490

This PR just moves `Theme` and the "follow system theme" settings to
egui and adds `RawInput.system_theme`.
A follow-up PR can then introduce the two separate `dark_mode_style` and
`light_mode_style` fields on `Options`.


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### Breaking changes

The options `follow_system_theme` and `default_theme` has been moved
from `eframe` into `egui::Options`, settable with `ctx.options_mut`

---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/eframe/src/web/app_runner.rs

@@ -38,18 +38,11 @@ impl AppRunner {
     ) -> Result<Self, String> {
         let painter = super::ActiveWebPainter::new(canvas, &web_options).await?;
 
-        let system_theme = if web_options.follow_system_theme {
-            super::system_theme()
-        } else {
-            None
-        };
-
         let info = epi::IntegrationInfo {
             web_info: epi::WebInfo {
                 user_agent: super::user_agent().unwrap_or_default(),
                 location: super::web_location(),
             },
-            system_theme,
             cpu_usage: None,
         };
         let storage = LocalStorage::default();
@@ -68,9 +61,6 @@ impl AppRunner {
             o.zoom_factor = 1.0;
         });
 
-        let theme = system_theme.unwrap_or(web_options.default_theme);
-        egui_ctx.set_visuals(theme.egui_visuals());
-
         let cc = epi::CreationContext {
             egui_ctx: egui_ctx.clone(),
             integration_info: info.clone(),
@@ -132,6 +122,7 @@ impl AppRunner {
             .entry(egui::ViewportId::ROOT)
             .or_default()
             .native_pixels_per_point = Some(super::native_pixels_per_point());
+        runner.input.raw.system_theme = super::system_theme();
 
         Ok(runner)
     }