diff --git a/sphinx/texinputs/sphinx.sty b/sphinx/texinputs/sphinx.sty
index f1896902e..c851acb53 100644
--- a/sphinx/texinputs/sphinx.sty
+++ b/sphinx/texinputs/sphinx.sty
@@ -733,7 +733,7 @@ will be set to white}%
        It does not seem to be available on your system.\MessageBreak
        Options for setting radii will be ignored}%
     \sphinxbuildwarning{nopict2e}%
-    \def\spx@boxes@fcolorbox@rounded{\spx@boxes@fcolorbox}%
+    \def\spx@boxes@fcolorbox@rounded{\spx@boxes@fcolorbox@rectangle}%
    }%
 }%
 \AtBeginDocument{%
diff --git a/sphinx/texinputs/sphinxlatexshadowbox.sty b/sphinx/texinputs/sphinxlatexshadowbox.sty
index 885509e37..e50e9946b 100644
--- a/sphinx/texinputs/sphinxlatexshadowbox.sty
+++ b/sphinx/texinputs/sphinxlatexshadowbox.sty
@@ -25,6 +25,7 @@
 
 % At 5.1.0 the code formerly here has been refactored and incorporated
 % into more powerful \spx@boxes@fcolorbox from sphinxpackageboxes.sty
+% (which not only has a @rectangle variant but also @rounded)
 \long\def\spx@ShadowFBox#1{%
   \leavevmode
   \setbox\z@\hbox{#1}%
diff --git a/sphinx/texinputs/sphinxpackageboxes.sty b/sphinx/texinputs/sphinxpackageboxes.sty
index 5d858fc42..7f6683ec1 100644
--- a/sphinx/texinputs/sphinxpackageboxes.sty
+++ b/sphinx/texinputs/sphinxpackageboxes.sty
@@ -51,8 +51,8 @@
 %%%%%%%%%%%%%%%%
 % "setup" macro
 %
-% It must be called prior to \spx@boxes@fcolorbox{@rounded,@insetshadow} for
-% initializing its parameters.
+% It must be called prior to \spx@boxes@fcolorbox for parameters of the latter
+% to be initialized.
 %
 % It also prepares \spx@boxes@fcolorbox to expand to one of
 % \spx@boxes@fcolorbox@rectangle or \spx@boxes@fcolorbox@rounded depending on