diff --git a/pyproject.toml b/pyproject.toml
index 9d4f342..d964fa3 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -64,7 +64,7 @@ sphinx = { version = "^5.0", optional = true }
 sphinx-autodoc-typehints = { version = "^1.12", optional = true }
 m2r2 = { version = "^0.3", optional = true }
 tomlkit = { version = "^0.11", optional = true }
-sphinx-rtd-theme = "^1.0.0"
+sphinx-rtd-theme = { version = "^1.0.0", optional = true }
 
 [tool.poetry.dev-dependencies]
 mypy = "^0.971"