Improve the search field styling in the sphinx13 theme (sphinx-doc#12591)

Author: timhoffm

doc/_themes/sphinx13/static/sphinx13.css

@@ -218,6 +218,16 @@ div.sphinxsidebar #searchbox {
     margin: 12px 0 20px 0;
 }
 
+div.sphinxsidebar #searchbox input[type="text"] {
+    border-radius: var(--admonition-radius) 0 0 var(--admonition-radius);
+}
+
+div.sphinxsidebar #searchbox input[type="submit"] {
+    border-radius: 0 var(--admonition-radius) var(--admonition-radius) 0;
+  color: white;
+    background: var(--colour-sphinx-blue);
+}
+
 div.footer {
     background-color: var(--colour-sphinx-blue);
     color: #ccc;