diff --git a/src/handler.rs b/src/handler.rs
index 536efd4..fdc3c9e 100644
--- a/src/handler.rs
+++ b/src/handler.rs
@@ -25,9 +25,6 @@ impl Default for RgbColors {
     }
 }
 
-/// Function to format a message before rendering.
-pub type MessageFormatter = dyn Fn(String) -> String;
-
 /**
 Create a custom [`MietteHandler`] from options.
 
@@ -58,7 +55,6 @@ pub struct MietteHandlerOpts {
     pub(crate) context_lines: Option<usize>,
     pub(crate) tab_width: Option<usize>,
     pub(crate) with_cause_chain: Option<bool>,
-    pub(crate) message_formatter: Option<Box<MessageFormatter>>,
 }
 
 impl MietteHandlerOpts {
@@ -168,11 +164,6 @@ impl MietteHandlerOpts {
         self
     }
 
-    pub fn message_formatter<F: MessageFormatter>(mut self, formatter: F) -> Self {
-        self.message_formatter = Some(Box::new(formatter));
-        self
-    }
-
     /// Builds a [`MietteHandler`] from this builder.
     pub fn build(self) -> MietteHandler {
         let graphical = self.is_graphical();
@@ -221,7 +212,11 @@ impl MietteHandlerOpts {
             } else {
                 ThemeStyles::none()
             };
-            let theme = self.theme.unwrap_or(GraphicalTheme { characters, styles });
+            let theme = self.theme.unwrap_or(GraphicalTheme {
+                characters,
+                styles,
+                tags: std::collections::HashMap::new(),
+            });
             let mut handler = GraphicalReportHandler::new()
                 .with_width(width)
                 .with_links(linkify)
@@ -242,9 +237,6 @@ impl MietteHandlerOpts {
             if let Some(w) = self.tab_width {
                 handler = handler.tab_width(w);
             }
-            if let Some(formatter) = self.message_formatter {
-                handler = handler.with_message_formatter(formatter);
-            }
             MietteHandler {
                 inner: Box::new(handler),
             }
diff --git a/src/handlers/graphical.rs b/src/handlers/graphical.rs
index 09e18e3..83d731c 100644
--- a/src/handlers/graphical.rs
+++ b/src/handlers/graphical.rs
@@ -6,9 +6,7 @@ use unicode_width::UnicodeWidthChar;
 use crate::diagnostic_chain::DiagnosticChain;
 use crate::handlers::theme::*;
 use crate::protocol::{Diagnostic, Severity};
-use crate::{
-    LabeledSpan, MessageFormatter, MietteError, ReportHandler, SourceCode, SourceSpan, SpanContents,
-};
+use crate::{LabeledSpan, MietteError, ReportHandler, SourceCode, SourceSpan, SpanContents};
 
 /**
 A [`ReportHandler`] that displays a given [`Report`](crate::Report) in a
@@ -32,7 +30,6 @@ pub struct GraphicalReportHandler {
     pub(crate) context_lines: usize,
     pub(crate) tab_width: usize,
     pub(crate) with_cause_chain: bool,
-    pub(crate) message_formatter: Option<Box<MessageFormatter>>,
 }
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
@@ -54,7 +51,6 @@ impl GraphicalReportHandler {
             context_lines: 1,
             tab_width: 4,
             with_cause_chain: true,
-            message_formatter: None,
         }
     }
 
@@ -68,7 +64,6 @@ impl GraphicalReportHandler {
             context_lines: 1,
             tab_width: 4,
             with_cause_chain: true,
-            message_formatter: None,
         }
     }
 
@@ -138,11 +133,6 @@ impl GraphicalReportHandler {
         self.context_lines = lines;
         self
     }
-
-    pub fn with_message_formatter<F: MessageFormatter>(mut self, formatter: F) -> Self {
-        self.message_formatter = Some(Box::new(formatter));
-        self
-    }
 }
 
 impl Default for GraphicalReportHandler {
@@ -226,9 +216,6 @@ impl GraphicalReportHandler {
             .subsequent_indent(&rest_indent);
         let mut message = diagnostic.to_string();
 
-        if let Some(formatter) = self.message_formatter {
-            message = formatter(message);
-        }
 
         writeln!(f, "{}", textwrap::fill(&message, opts))?;
 
diff --git a/src/handlers/theme.rs b/src/handlers/theme.rs
index 1f5236a..ca89800 100644
--- a/src/handlers/theme.rs
+++ b/src/handlers/theme.rs
@@ -1,5 +1,6 @@
 use is_terminal::IsTerminal;
 use owo_colors::Style;
+use std::collections::HashMap;
 
 /**
 Theme used by [`GraphicalReportHandler`](crate::GraphicalReportHandler) to
@@ -18,6 +19,8 @@ pub struct GraphicalTheme {
     pub characters: ThemeCharacters,
     /// Styles to be used for painting.
     pub styles: ThemeStyles,
+    /// Tags to be used for styling.
+    pub tags: HashMap<String, Style>,
 }
 
 impl GraphicalTheme {
@@ -26,6 +29,7 @@ impl GraphicalTheme {
         Self {
             characters: ThemeCharacters::ascii(),
             styles: ThemeStyles::ansi(),
+            tags: HashMap::new(),
         }
     }
 
@@ -41,6 +45,7 @@ impl GraphicalTheme {
         Self {
             characters: ThemeCharacters::unicode(),
             styles: ThemeStyles::ansi(),
+            tags: HashMap::new(),
         }
     }
 
@@ -50,6 +55,7 @@ impl GraphicalTheme {
         Self {
             characters: ThemeCharacters::unicode(),
             styles: ThemeStyles::none(),
+            tags: HashMap::new(),
         }
     }
 
@@ -62,6 +68,7 @@ impl GraphicalTheme {
         Self {
             characters: ThemeCharacters::ascii(),
             styles: ThemeStyles::none(),
+            tags: HashMap::new(),
         }
     }
 }