eframe: Don't follow system theme by default (#1941)

I have gone back and forth on this a bit, but I think the arguments
AGAINST following the system theme are many:

* `dark-light` is a big dependency with problems on Linux.
* Many people prefer the dark mode and ask how to set it as the default
  (even though they are using light mode in their OS).
* A developer may be surprised when the app changes theme when
  they run it on another computer.

So, the path of least surprise is to make this an opt-in feature
with dark mode as the default mode.

On native, you add the `dark-light` feature to enable it.
On web, you set `WebOptions::follow_system_theme`.

crates/eframe/CHANGELOG.md

@@ -7,8 +7,8 @@ NOTE: [`egui-winit`](../egui-winit/CHANGELOG.md), [`egui_glium`](../egui_glium/C
 ## Unreleased
 * MSRV (Minimum Supported Rust Version) is now `1.61.0` ([#1846](https://github.com/emilk/egui/pull/1846)).
 * Added `wgpu` rendering backed ([#1564](https://github.com/emilk/egui/pull/1564)):
-  * Added features "wgpu" and "glow"
-  * Added `NativeOptions::renderer` to switch between the rendering backends
+  * Added features `wgpu` and `glow`.
+  * Added `NativeOptions::renderer` to switch between the rendering backends.
 * `egui_glow`: remove calls to `gl.get_error` in release builds to speed up rendering ([#1583](https://github.com/emilk/egui/pull/1583)).
 * Added `App::post_rendering` for e.g. reading the framebuffer ([#1591](https://github.com/emilk/egui/pull/1591)).
 * Use `Arc` for `glow::Context` instead of `Rc` ([#1640](https://github.com/emilk/egui/pull/1640)).
@@ -21,14 +21,14 @@ NOTE: [`egui-winit`](../egui-winit/CHANGELOG.md), [`egui_glium`](../egui_glium/C
 * Added ability to read window position and size with `frame.info().window_info` ([#1617](https://github.com/emilk/egui/pull/1617)).
 * Allow running on native without hardware accelerated rendering. Change with `NativeOptions::hardware_acceleration` ([#1681](https://github.com/emilk/egui/pull/1681), [#1693](https://github.com/emilk/egui/pull/1693)).
 * Fixed window position persistence ([#1745](https://github.com/emilk/egui/pull/1745)).
-* `dark-light` (dark mode detection) is now enabled by default on Mac and Windows ([#1726](https://github.com/emilk/egui/pull/1726)).
 * Fixed mouse cursor change on Linux ([#1747](https://github.com/emilk/egui/pull/1747)).
 * Added `Frame::set_visible` ([#1808](https://github.com/emilk/egui/pull/1808)).
 * Added fullscreen support ([#1866](https://github.com/emilk/egui/pull/1866)).
 
 #### Web:
-* Added option to select WebGL version ([#1803](https://github.com/emilk/egui/pull/1803)).
 * Added ability to stop/re-run web app from JavaScript. ⚠️ You need to update your CSS with `html, body: { height: 100%; width: 100%; }` ([#1803](https://github.com/emilk/egui/pull/1650)).
+* Added `WebOptions::follow_system_theme` and `WebOptions::default_theme` ([#1726](https://github.com/emilk/egui/pull/1726)).
+* Added option to select WebGL version ([#1803](https://github.com/emilk/egui/pull/1803)).
 
 
 

crates/eframe/Cargo.toml

@@ -20,7 +20,7 @@ all-features = true
 
 
 [features]
-default = ["dark-light", "default_fonts", "glow"]
+default = ["default_fonts", "glow"]
 
 ## Detect dark mode system preference using [`dark-light`](https://docs.rs/dark-light).
 ##

crates/eframe/src/epi.rs

@@ -259,7 +259,7 @@ pub struct NativeOptions {
     /// What rendering backend to use.
     pub renderer: Renderer,
 
-    /// If the `dark-light` feature is enabled:
+    /// Only used if the `dark-light` feature is enabled:
     ///
     /// Try to detect and follow the system preferred setting for dark vs light mode.
     ///
@@ -272,7 +272,7 @@ pub struct NativeOptions {
     /// Which theme to use in case [`Self::follow_system_theme`] is `false`
     /// or the `dark-light` feature is disabled.
     ///
-    /// Default: `Theme::Dark`.
+    /// Default: [`Theme::Dark`].
     pub default_theme: Theme,
 
     /// This controls what happens when you close the main eframe window.
@@ -351,7 +351,7 @@ pub struct WebOptions {
     ///
     /// See also [`Self::default_theme`].
     ///
-    /// Default: `true`.
+    /// Default: `false`.
     pub follow_system_theme: bool,
 
     /// Which theme to use in case [`Self::follow_system_theme`] is `false`