diff --git a/sphinx/themes/agogo/static/agogo.css_t b/sphinx/themes/agogo/static/agogo.css_t
index 4c7eb378b..8646e2ddf 100644
--- a/sphinx/themes/agogo/static/agogo.css_t
+++ b/sphinx/themes/agogo/static/agogo.css_t
@@ -223,6 +223,10 @@ div.document .descname {
   font-weight: bold;
 }
 
+div.document .sig-paren {
+    font-size: larger;
+}
+
 div.document .docutils.literal {
   background-color: #eeeeec;
   padding: 1px;
diff --git a/sphinx/themes/basic/static/basic.css_t b/sphinx/themes/basic/static/basic.css_t
index a3255ebd4..907ede7a7 100644
--- a/sphinx/themes/basic/static/basic.css_t
+++ b/sphinx/themes/basic/static/basic.css_t
@@ -406,6 +406,10 @@ dl.glossary dt {
     font-size: 1.3em;
 }
 
+.sig-paren {
+    font-size: larger;
+}
+
 .versionmodified {
     font-style: italic;
 }
diff --git a/sphinx/themes/epub/static/epub.css b/sphinx/themes/epub/static/epub.css
index 6c8ff5e77..eec7a1478 100644
--- a/sphinx/themes/epub/static/epub.css
+++ b/sphinx/themes/epub/static/epub.css
@@ -322,6 +322,10 @@ dl.glossary dt {
     font-size: 130%;
 }
 
+.sig-paren {
+    font-size: larger;
+}
+
 .versionmodified {
     font-style: italic;
 }
diff --git a/sphinx/themes/scrolls/static/scrolls.css_t b/sphinx/themes/scrolls/static/scrolls.css_t
index c335f167e..a2d3a7c24 100644
--- a/sphinx/themes/scrolls/static/scrolls.css_t
+++ b/sphinx/themes/scrolls/static/scrolls.css_t
@@ -305,6 +305,10 @@ dt .descname {
     margin-right: 4px;
 }
 
+dt .sig-paren {
+    font-size: larger;
+}
+
 dt .descname, dt .descclassname {
     padding: 0;
     background: transparent;
diff --git a/sphinx/themes/traditional/static/traditional.css_t b/sphinx/themes/traditional/static/traditional.css_t
index cbea798c3..10fbb2d17 100644
--- a/sphinx/themes/traditional/static/traditional.css_t
+++ b/sphinx/themes/traditional/static/traditional.css_t
@@ -659,6 +659,10 @@ h1 tt, h2 tt, h3 tt, h4 tt, h5 tt, h6 tt {
     font-size: 1.3em;
 }
 
+.sig-paren {
+    font-size: larger;
+}
+
 .versionmodified {
     font-style: italic;
 }
diff --git a/sphinx/writers/html.py b/sphinx/writers/html.py
index 431385cb9..ecd02a73c 100644
--- a/sphinx/writers/html.py
+++ b/sphinx/writers/html.py
@@ -130,7 +130,7 @@ class HTMLTranslator(BaseTranslator):
         self.body.append('</tt>')
 
     def visit_desc_parameterlist(self, node):
-        self.body.append('<big>(</big>')
+        self.body.append('<span class="sig-paren">(</span>')
         self.first_param = 1
         self.optional_param_level = 0
         # How many required parameters are left.
@@ -138,7 +138,7 @@ class HTMLTranslator(BaseTranslator):
                                          for c in node.children])
         self.param_separator = node.child_text_separator
     def depart_desc_parameterlist(self, node):
-        self.body.append('<big>)</big>')
+        self.body.append('<span class="sig-paren">)</span>')
 
     # If required parameters are still to come, then put the comma after
     # the parameter.  Otherwise, put the comma before.  This ensures that