diff --git a/book/source/_static/epub/css/custom.css b/book/source/_static/epub/css/custom.css
new file mode 100644
index 0000000..40ca147
--- /dev/null
+++ b/book/source/_static/epub/css/custom.css
@@ -0,0 +1,13 @@
+/*
+SPDX-FileCopyrightText: 2023 The "Notes on OpenPGP" project
+SPDX-License-Identifier: CC0-1.0
+*/
+
+div.admonition {
+  background-color: lightgray;
+}
+
+div.warning,
+div.admonition-warning {
+  background-color: palevioletred;
+}
\ No newline at end of file
diff --git a/book/source/conf.py b/book/source/conf.py
index 96f5eb4..abacb6f 100644
--- a/book/source/conf.py
+++ b/book/source/conf.py
@@ -24,7 +24,12 @@ source_suffix = ['.md', '.rst']
 templates_path = ['_templates']
 exclude_patterns = []
 
+# -- Options for EPUB output -------------------------------------------------
+# https://www.sphinx-doc.org/en/master/usage/configuration.html#options-for-epub-output
 
+epub_css_files = [
+    'epub/css/custom.css'
+]
 
 # -- Options for HTML output -------------------------------------------------
 # https://www.sphinx-doc.org/en/master/usage/configuration.html#options-for-html-output