diff --git a/crates/mdbook-html/src/html_handlebars/hbs_renderer.rs b/crates/mdbook-html/src/html_handlebars/hbs_renderer.rs
index 1c7f01001b..f7404c53f3 100644
--- a/crates/mdbook-html/src/html_handlebars/hbs_renderer.rs
+++ b/crates/mdbook-html/src/html_handlebars/hbs_renderer.rs
@@ -271,8 +271,6 @@ impl HtmlHandlebars {
                 no_section_label: html_config.no_section_label,
             }),
         );
-        // TODO: remove theme_option in 0.5, it is not needed.
-        handlebars.register_helper("theme_option", Box::new(helpers::theme::theme_option));
     }
 
     fn emit_redirects(
@@ -558,7 +556,6 @@ fn make_data(
         );
     }
 
-    // TODO: remove default_theme in 0.5, it is not needed.
     let default_theme = match html_config.default_theme {
         Some(ref theme) => theme.to_lowercase(),
         None => "light".to_string(),
diff --git a/crates/mdbook-html/src/html_handlebars/helpers/mod.rs b/crates/mdbook-html/src/html_handlebars/helpers/mod.rs
index 720704f3e1..963e8f10af 100644
--- a/crates/mdbook-html/src/html_handlebars/helpers/mod.rs
+++ b/crates/mdbook-html/src/html_handlebars/helpers/mod.rs
@@ -1,3 +1,2 @@
 pub(crate) mod resources;
-pub(crate) mod theme;
 pub(crate) mod toc;
diff --git a/crates/mdbook-html/src/html_handlebars/helpers/resources.rs b/crates/mdbook-html/src/html_handlebars/helpers/resources.rs
index 01b9738f12..f3cf9cd497 100644
--- a/crates/mdbook-html/src/html_handlebars/helpers/resources.rs
+++ b/crates/mdbook-html/src/html_handlebars/helpers/resources.rs
@@ -23,7 +23,7 @@ impl HelperDef for ResourceHelper {
     ) -> Result<(), RenderError> {
         let param = h.param(0).and_then(|v| v.value().as_str()).ok_or_else(|| {
             RenderErrorReason::Other(
-                "Param 0 with String type is required for theme_option helper.".to_owned(),
+                "Param 0 with String type is required for resource helper.".to_owned(),
             )
         })?;
 
diff --git a/crates/mdbook-html/src/html_handlebars/helpers/theme.rs b/crates/mdbook-html/src/html_handlebars/helpers/theme.rs
deleted file mode 100644
index 120a8d2ff4..0000000000
--- a/crates/mdbook-html/src/html_handlebars/helpers/theme.rs
+++ /dev/null
@@ -1,38 +0,0 @@
-use handlebars::{
-    Context, Handlebars, Helper, Output, RenderContext, RenderError, RenderErrorReason,
-};
-use log::trace;
-
-pub(crate) fn theme_option(
-    h: &Helper<'_>,
-    _r: &Handlebars<'_>,
-    ctx: &Context,
-    rc: &mut RenderContext<'_, '_>,
-    out: &mut dyn Output,
-) -> Result<(), RenderError> {
-    trace!("theme_option (handlebars helper)");
-
-    let param = h.param(0).and_then(|v| v.value().as_str()).ok_or_else(|| {
-        RenderErrorReason::ParamTypeMismatchForName(
-            "theme_option",
-            "0".to_owned(),
-            "string".to_owned(),
-        )
-    })?;
-
-    let default_theme = rc.evaluate(ctx, "@root/default_theme")?;
-    let default_theme_name = default_theme.as_json().as_str().ok_or_else(|| {
-        RenderErrorReason::ParamTypeMismatchForName(
-            "theme_option",
-            "default_theme".to_owned(),
-            "string".to_owned(),
-        )
-    })?;
-
-    out.write(param)?;
-    if param.to_lowercase() == default_theme_name.to_lowercase() {
-        out.write(" (default)")?;
-    }
-
-    Ok(())
-}