diff --git a/pyproject.toml b/pyproject.toml
index 5ebf32a..c6fb8cc 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -20,7 +20,7 @@ sphinx = "^8.0.2"
 sphinx-autodoc-typehints = "^2.2.3"
 
 [project.optional-dependencies]
-documentation = ["sphinx-autodoc-typehints"]
+documentation = ["sphinx-autodoc-typehints", "sphinx_rtd_theme"]
 
 [tool.pytest.ini_options]
 testpaths = ["tests"]