Rollup merge of #92629 - jsha:theme-picker-local-only-2, r=GuillaumeGomez

Pick themes on settings page, not every page

This hides the paintbrush icon on most pages by default, in preference for the settings on the settings page.  When loading from a local file, and not in mobile view, continue to show the theme picker. That's because some browsers limit access to localStorage from file:/// URLs, so choosing a theme from settings.html doesn't take effect.

Fixes #84539
Part of #59840

r? `@GuillaumeGomez`

Demo: https://rustdoc.crud.net/jsha/theme-picker-local-only-2/std/io/trait.Read.html

src/librustdoc/html/render/mod.rs

@@ -422,6 +422,12 @@ fn settings(root_path: &str, suffix: &str, theme_names: Vec<String>) -> Result<S
             "Theme preferences",
             vec![
                 Setting::from(("use-system-theme", "Use system theme", true)),
+                Setting::Select {
+                    js_data_name: "theme",
+                    description: "Theme",
+                    default_value: "light",
+                    options: theme_names.clone(),
+                },
                 Setting::Select {
                     js_data_name: "preferred-dark-theme",
                     description: "Preferred dark theme",