diff --git a/docker/Dockerfile_ubuntu_18.04 b/docker/Dockerfile_ubuntu_18.04
index 930f670a3d355e1aaa084c50e3fca6812332b0da..1beae422bcf15f3e7e8eb0c1295bec20a245ff7c 100644
--- a/docker/Dockerfile_ubuntu_18.04
+++ b/docker/Dockerfile_ubuntu_18.04
@@ -14,6 +14,7 @@ RUN apt-get update && \
         python3-sphinx \
         python3-numpydoc \
         python3-sphinx-gallery \
+        python3-sphinx-rtd-theme \
         python3-matplotlib \
         python3-pil \
         python3-tk \