diff --git a/src/syntax.rs b/src/syntax.rs
index 780b8d1..42b25b7 100644
--- a/src/syntax.rs
+++ b/src/syntax.rs
@@ -92,6 +92,7 @@ impl LanguageRegistry {
     fn load(&self, name: &str) -> Option<&'static LanguageConfig> {
         let lang_dir = self.syntax_root.join(name);
         if !lang_dir.join(format!("{name}.so")).exists() {
+            warn!("No grammar found for language {name}");
             return None;
         }
         match load_language(name, &lang_dir, &self.syntax_root) {
@@ -229,14 +230,18 @@ fn highlight_code(
     light: Option<&str>,
     dark: Option<&str>,
 ) -> Option<Content> {
-    let scopes = crate::themes::shared_scopes(light?, dark?)?;
+    let (lang, light, dark) = (lang?, light?, dark?);
+    let Some(scopes) = crate::themes::shared_scopes(light, dark) else {
+        warn!("Unknown theme in pair (light={light}, dark={dark})");
+        return None;
+    };
     let registry = registry_for(scopes)?;
-    let language = registry.language(lang?)?;
+    let language = registry.language(lang)?;
 
     let rope = Rope::from_str(source);
     let slice = rope.slice(..);
     let loader = registry.as_ref();
-    let syntax = Syntax::new(slice, language, Duration::from_millis(500), loader).ok()?;
+    let syntax = Syntax::new(slice, language, Duration::from_secs(3), loader).ok()?;
     let mut hl = Highlighter::new(&syntax, slice, loader, 0..source.len() as u32);
 
     let src = source.as_bytes();
diff --git a/src/themes.rs b/src/themes.rs
index 2ed442e..aa61a64 100644
--- a/src/themes.rs
+++ b/src/themes.rs
@@ -82,7 +82,13 @@ impl ThemeRegistry {
 
         let themes = helix_themes
             .into_iter()
-            .flat_map(|(name, table)| Theme::from_toml(table).map(|t| (name, t)))
+            .flat_map(|(name, table)| match Theme::from_toml(table) {
+                Some(theme) => Some((name, theme)),
+                None => {
+                    warn!("Theme {name} is missing an `ui.background`, skipping it");
+                    None
+                }
+            })
             .collect();
 
         Self { themes }
@@ -104,8 +110,13 @@ impl Color {
     fn from_palette(color: &str, palette: &HashMap<String, Color>) -> Option<Self> {
         if color.starts_with('#') {
             Self::from_str(color)
+        } else if let Some(color) = palette.get(color) {
+            Some(color.clone())
         } else {
-            palette.get(color).cloned()
+            // Neither a hex literal nor a palette entry; assume a named CSS
+            // colour and pass it through verbatim.
+            warn!("Unresolved colour {color}, passing it through as a literal");
+            Some(Self(color.into()))
         }
     }
 }
@@ -282,14 +293,21 @@ fn generate_css(scopes: &[&str], theme: &Theme) -> String {
             groups.entry(style).or_default().push(scope);
         }
     }
-    rules.extend(groups.iter().map(|(style, scopes)| {
-        let selectors = scopes
-            .iter()
-            .map(|s| scope_to_selector(s))
-            .collect::<Vec<_>>()
-            .join(",");
-        format!("{}{{{}}}", selectors, style_declarations(style))
-    }));
+
+    let mut group_rules: Vec<String> = groups
+        .iter()
+        .map(|(style, scopes)| {
+            let selectors = scopes
+                .iter()
+                .map(|s| scope_to_selector(s))
+                .collect::<Vec<_>>()
+                .join(",");
+            format!("{}{{{}}}", selectors, style_declarations(style))
+        })
+        .collect();
+    // Sort so the emitted stylesheet is deterministic across runs.
+    group_rules.sort();
+    rules.extend(group_rules);
 
     rules.join("\n")
 }
@@ -308,13 +326,8 @@ fn theme_css(light: String, dark: String) -> String {
         return String::new();
     };
 
-    let mut shared: Vec<&str> = light_theme
-        .styles
-        .keys()
-        .filter(|k| dark_theme.styles.contains_key(*k))
-        .map(String::as_str)
-        .collect();
-    shared.sort();
+    let shared = shared_scopes(&light, &dark).unwrap_or_default();
+    let shared: Vec<&str> = shared.iter().map(String::as_str).collect();
 
     let light_css = generate_css(&shared, light_theme);
     let dark_css = generate_css(&shared, dark_theme);