Load alt default theme if true color is not supported

* Move `runtime/themes/base16_default_terminal.toml` to
  `base16_theme.toml` alongside `theme.toml`
* Use `terminfo` crate to detect whether the terminal supports true
  color and, if the user has no theme configured and their terminal does
  not support true color, load the alt default theme instead of the
  normal default.

Remove `terminfo` dependency, use `COLORTERM` env instead

Prevent user from switching to an unsupported theme

Add `true-color-override` option

If the terminal is wrongly detected to not support true color,
`true-color-override = true` will override the detection.

Rename `true-color-override` to `true-color`

8 files changed, 62 insertions, 20 deletions

diff --git a/runtime/themes/base16_terminal.toml b/base16_theme.toml
index bf3c73f1..bf3c73f1 100644
--- a/runtime/themes/base16_terminal.toml
+++ b/base16_theme.toml
diff --git a/book/src/configuration.md b/book/src/configuration.md
index 2ed48d51..33a933b2 100644
--- a/book/src/configuration.md
+++ b/book/src/configuration.md
@@ -23,6 +23,7 @@ To override global configuration parameters, create a `config.toml` file located
 | `idle-timeout` | Time in milliseconds since last keypress before idle timers trigger. Used for autocompletion, set to 0 for instant. | `400` |
 | `completion-trigger-len` | The min-length of word under cursor to trigger autocompletion | `2` |
 | `auto-info` | Whether to display infoboxes | `true` |
+| `true-color` | Set to `true` to override automatic detection of terminal truecolor support in the event of a false negative. | `false` |
 
 `[editor.filepicker]` section of the config. Sets options for file picker and global search. All but the last key listed in the default file-picker configuration below are IgnoreOptions: whether hidden files and files listed within ignore files are ignored by (not visible in) the helix file picker and global search. There is also one other key, `max-depth` available, which is not defined by default.
 
diff --git a/helix-term/src/application.rs b/helix-term/src/application.rs
index 90330751..3e0b6d59 100644
--- a/helix-term/src/application.rs
+++ b/helix-term/src/application.rs
@@ -76,17 +76,27 @@ impl Application {
             None => Ok(def_lang_conf),
         };
 
-        let theme = if let Some(theme) = &config.theme {
-            match theme_loader.load(theme) {
-                Ok(theme) => theme,
-                Err(e) => {
-                    log::warn!("failed to load theme `{}` - {}", theme, e);
+        let true_color = config.editor.true_color || crate::true_color();
+        let theme = config
+            .theme
+            .as_ref()
+            .and_then(|theme| {
+                theme_loader
+                    .load(theme)
+                    .map_err(|e| {
+                        log::warn!("failed to load theme `{}` - {}", theme, e);
+                        e
+                    })
+                    .ok()
+                    .filter(|theme| (true_color || theme.is_16_color()))
+            })
+            .unwrap_or_else(|| {
+                if true_color {
                     theme_loader.default()
+                } else {
+                    theme_loader.base16_default()
                 }
-            }
-        } else {
-            theme_loader.default()
-        };
+            });
 
         let syn_loader_conf: helix_core::syntax::Configuration = lang_conf
             .and_then(|conf| conf.try_into())
diff --git a/helix-term/src/commands.rs b/helix-term/src/commands.rs
index 0ae225e9..22c23043 100644
--- a/helix-term/src/commands.rs
+++ b/helix-term/src/commands.rs
@@ -2366,8 +2366,18 @@ pub mod cmd {
         args: &[Cow<str>],
         _event: PromptEvent,
     ) -> anyhow::Result<()> {
-        let theme = args.first().context("theme not provided")?;
-        cx.editor.set_theme_from_name(theme)
+        let theme = args.first().context("Theme not provided")?;
+        let theme = cx
+            .editor
+            .theme_loader
+            .load(theme)
+            .with_context(|| format!("Failed setting theme {}", theme))?;
+        let true_color = cx.editor.config.true_color || crate::true_color();
+        if !(true_color || theme.is_16_color()) {
+            bail!("Unsupported theme: theme requires true color support");
+        }
+        cx.editor.set_theme(theme);
+        Ok(())
     }
 
     fn yank_main_selection_to_clipboard(
diff --git a/helix-term/src/lib.rs b/helix-term/src/lib.rs
index f5e3a8cd..4fe76cd5 100644
--- a/helix-term/src/lib.rs
+++ b/helix-term/src/lib.rs
@@ -9,3 +9,9 @@ pub mod config;
 pub mod job;
 pub mod keymap;
 pub mod ui;
+
+fn true_color() -> bool {
+    std::env::var("COLORTERM")
+        .map(|v| matches!(v.as_str(), "truecolor" | "24bit"))
+        .unwrap_or(false)
+}
diff --git a/helix-term/src/ui/mod.rs b/helix-term/src/ui/mod.rs
index cdf42311..f57e2e2b 100644
--- a/helix-term/src/ui/mod.rs
+++ b/helix-term/src/ui/mod.rs
@@ -186,6 +186,7 @@ pub mod completers {
             &helix_core::config_dir().join("themes"),
         ));
         names.push("default".into());
+        names.push("base16_default".into());
 
         let mut names: Vec<_> = names
             .into_iter()
diff --git a/helix-view/src/editor.rs b/helix-view/src/editor.rs
index dcbcbe4f..753defa1 100644
--- a/helix-view/src/editor.rs
+++ b/helix-view/src/editor.rs
@@ -105,6 +105,8 @@ pub struct Config {
     /// Whether to display infoboxes. Defaults to true.
     pub auto_info: bool,
     pub file_picker: FilePickerConfig,
+    /// Set to `true` to override automatic detection of terminal truecolor support in the event of a false negative. Defaults to `false`.
+    pub true_color: bool,
 }
 
 #[derive(Debug, Copy, Clone, PartialEq, Eq, Deserialize)]
@@ -137,6 +139,7 @@ impl Default for Config {
             completion_trigger_len: 2,
             auto_info: true,
             file_picker: FilePickerConfig::default(),
+            true_color: false,
         }
     }
 }
@@ -265,15 +268,6 @@ impl Editor {
         self._refresh();
     }
 
-    pub fn set_theme_from_name(&mut self, theme: &str) -> anyhow::Result<()> {
-        let theme = self
-            .theme_loader
-            .load(theme.as_ref())
-            .with_context(|| format!("failed setting theme `{}`", theme))?;
-        self.set_theme(theme);
-        Ok(())
-    }
-
     /// Refreshes the language server for a given document
     pub fn refresh_language_server(&mut self, doc_id: DocumentId) -> Option<()> {
         let doc = self.documents.get_mut(&doc_id)?;
diff --git a/helix-view/src/theme.rs b/helix-view/src/theme.rs
index 47fc6262..ddcdad99 100644
--- a/helix-view/src/theme.rs
+++ b/helix-view/src/theme.rs
@@ -15,6 +15,10 @@ pub use crate::graphics::{Color, Modifier, Style};
 pub static DEFAULT_THEME: Lazy<Theme> = Lazy::new(|| {
     toml::from_slice(include_bytes!("../../theme.toml")).expect("Failed to parse default theme")
 });
+pub static BASE16_DEFAULT_THEME: Lazy<Theme> = Lazy::new(|| {
+    toml::from_slice(include_bytes!("../../base16_theme.toml"))
+        .expect("Failed to parse base 16 default theme")
+});
 
 #[derive(Clone, Debug)]
 pub struct Loader {
@@ -35,6 +39,9 @@ impl Loader {
         if name == "default" {
             return Ok(self.default());
         }
+        if name == "base16_default" {
+            return Ok(self.base16_default());
+        }
         let filename = format!("{}.toml", name);
 
         let user_path = self.user_dir.join(&filename);
@@ -74,6 +81,11 @@ impl Loader {
     pub fn default(&self) -> Theme {
         DEFAULT_THEME.clone()
     }
+
+    /// Returns the alternative 16-color default theme
+    pub fn base16_default(&self) -> Theme {
+        BASE16_DEFAULT_THEME.clone()
+    }
 }
 
 #[derive(Clone, Debug)]
@@ -154,6 +166,14 @@ impl Theme {
     pub fn find_scope_index(&self, scope: &str) -> Option<usize> {
         self.scopes().iter().position(|s| s == scope)
     }
+
+    pub fn is_16_color(&self) -> bool {
+        self.styles.iter().all(|(_, style)| {
+            [style.fg, style.bg]
+                .into_iter()
+                .all(|color| !matches!(color, Some(Color::Rgb(..))))
+        })
+    }
 }
 
 struct ThemePalette {