--- a/eric6/UI/Previewers/PreviewerHTML.py	Sat Apr 11 14:50:48 2020 +0200
+++ b/eric6/UI/Previewers/PreviewerHTML.py	Sat Apr 11 19:08:37 2020 +0200
@@ -743,12 +743,37 @@
                 " src='https://unpkg.com/mermaid@8/dist/mermaid.min.js'>\n"
                 "</script>\n"
             )
+            if e5App().usesDarkPalette():
+                mermaid_initialize = (
+                    "<script>mermaid.initialize({"
+                    "theme: 'dark', "
+                    "startOnLoad:true"
+                    "});</script>"
+                )
+            else:
+                mermaid_initialize = (
+                    "<script>mermaid.initialize({"
+                    "theme: 'default', "
+                    "startOnLoad:true"
+                    "});</script>"
+                )
         else:
             mermaid = ""
+            mermaid_initialize = ""
         
         htmlFormat = Preferences.getEditor("PreviewMarkdownHTMLFormat").lower()
         body = markdown.markdown(text, extensions=extensions,
                                  output_format=htmlFormat.lower())
+        if e5App().usesDarkPalette():
+            style = (
+                PreviewerHTMLStyles.css_markdown_dark +
+                PreviewerHTMLStyles.css_pygments_dark
+            )
+        else:
+            style = (
+                PreviewerHTMLStyles.css_markdown_light +
+                PreviewerHTMLStyles.css_pygments_light
+            )
         
         if htmlFormat == "xhtml1":
             head = (
@@ -777,12 +802,8 @@
             '''</style>\n'''
             '''</head>\n'''
             '''<body>\n'''
-        ).format(
-            mathjax, mermaid,
-            PreviewerHTMLStyles.css_markdown +
-            PreviewerHTMLStyles.css_pygments
-        )
+        ).format(mathjax, mermaid, style)
         
         foot = '''\n</body>\n</html>\n'''
         
-        return head + body + foot
+        return head + body + mermaid_initialize + foot