Contents is not displayed and KIOExec error is shown instead. Seems like the problem is rejecting data:// URLs which are used by QWebEnginePage::setHtml(). The following patch makes it work.

diff --git a/lib/adium-theme-view.cpp b/lib/adium-theme-view.cpp
index 3b37285..3819788 100644
--- a/lib/adium-theme-view.cpp
+++ b/lib/adium-theme-view.cpp
@@ -68,6 +68,8 @@ bool AdiumThemePage::acceptNavigationRequest(const QUrl &url, QWebEnginePage::Na
         Q_EMIT nextConversation();
     } else if (url.fragment() == QLatin1String("x-prevConversation")) {
         Q_EMIT prevConversation();
+    } else if (url.scheme() == QLatin1String("data")) {
+        return true;
     } else {
         QDesktopServices::openUrl(url);
     }