rustdoc: Provide a --default-theme THEME option

This is a fairly simple special case of --default-eetting.  We must
set both "theme" and "use-system-theme".

Providing it separately enables us to document a way to set the theme
without expoosing the individual settings keywords, which are quite
complex.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

src/librustdoc/config.rs

@@ -378,6 +378,17 @@ impl Options {
         };
 
         let mut default_settings: Vec<Vec<(String, String)>> = vec![
+            matches
+                .opt_str("default-theme")
+                .iter()
+                .map(|theme| {
+                    vec![
+                        ("use-system-theme".to_string(), "false".to_string()),
+                        ("theme".to_string(), theme.to_string()),
+                    ]
+                })
+                .flatten()
+                .collect(),
             matches
                 .opt_strs("default-setting")
                 .iter()

src/librustdoc/lib.rs

@@ -269,6 +269,16 @@ fn opts() -> Vec<RustcOptGroup> {
                 "sort modules by where they appear in the program, rather than alphabetically",
             )
         }),
+        unstable("default-theme", |o| {
+            o.optopt(
+                "",
+                "default-theme",
+                "Set the default theme.  THEME should be the theme name, generally lowercase. \
+                 If an unknown default theme is specified, the builtin default is used. \
+                 The set of themes, and the rustdoc built-in default is not stable.",
+                "THEME",
+            )
+        }),
         unstable("default-setting", |o| {
             o.optmulti(
                 "",