diff --git a/Makefile b/Makefile
index 68a7642..a632c59 100644
--- a/Makefile
+++ b/Makefile
@@ -173,9 +173,6 @@ endif
 ifdef EDX_PLATFORM_VERSION
 	openedx_build_args += --build-arg="EDX_PLATFORM_VERSION=$(EDX_PLATFORM_VERSION)"
 endif
-ifdef THEMES
-	openedx_build_args += --build-arg="THEMES=$(THEMES)"
-endif
 
 build-openedx: ## Build the Open edX docker image
 	docker build -t regis/openedx:latest -t regis/openedx:hawthorn $(openedx_build_args) openedx/
diff --git a/README.md b/README.md
index 3f1b15c..f417224 100644
--- a/README.md
+++ b/README.md
@@ -278,9 +278,9 @@ Comprehensive theming is enabled by default. Put your themes in `openedx/themes`
         mycustomtheme2/
             ...
 
-Then you must rebuild the openedx Docker image and indicate which themes you want to build:
+Then you must rebuild the openedx Docker image:
 
-    make build-openedx THEMES=mycustomtheme1,mycustomtheme2,dark-theme
+    make build-openedx
 
 Make sure the assets can be served by the web server:
 
diff --git a/openedx/Dockerfile b/openedx/Dockerfile
index eafca4b..84021bf 100644
--- a/openedx/Dockerfile
+++ b/openedx/Dockerfile
@@ -80,9 +80,7 @@ COPY settings/cms/*.py /openedx/config/universal/cms/
 
 # Collect production assets. By default, only assets from the default theme
 # will be processed. This makes the docker image lighter and faster to build.
-# To compile assets from other themes, build the image with the THEMES
-# argument: THEMES=dark-theme,edx.org
-# Custom themes added to /openedx/themes can also be compiled.
+# Only the custom themes added to /openedx/themes will be compiled.
 # Here, we don't run "paver update_assets" which is slow, compiles all themes
 # and requires a complex settings file. Instead, we decompose the commands
 # and run each one individually to collect the production static assets to
@@ -95,9 +93,8 @@ COPY settings/cms/*.py /openedx/config/universal/cms/
 RUN xmodule_assets common/static/xmodule \
     && python -c "import pavelib.assets; pavelib.assets.process_npm_assets()" \ 
     && STATIC_ROOT_LMS=/openedx/data/staticfiles STATIC_ROOT_CMS=/openedx/data/staticfiles/studio NODE_ENV=production ./node_modules/.bin/webpack --config=webpack.prod.config.js
-ARG THEMES=open-edx
 COPY ./themes/ /openedx/themes/
-RUN paver compile_sass --theme-dirs=/openedx/edx-platform/themes,/openedx/themes --themes=$THEMES \
+RUN paver compile_sass --theme-dirs=/openedx/edx-platform/themes,/openedx/themes --themes=open-edx,$(ls /openedx/themes | paste -s -d, -) \
     && python -c "import pavelib.assets; pavelib.assets.collect_assets(['lms', 'cms'], 'universal.assets')"
 
 # service variant is "lms" or "cms"