Found another form that's not a pure-form yet.

The add contact form.

css/converse.css

@@ -1720,7 +1720,7 @@
         background-color: #DCEAC5; }
   #conversejs #controlbox .add-xmpp-contact {
     background: none;
-    padding: 5px; }
+    padding: 1em; }
     #conversejs #controlbox .add-xmpp-contact input {
       margin: 0 0 1rem;
       width: 100%; }
@@ -1786,7 +1786,7 @@
     height: 25px;
     margin: 0 0 0.5em 7px;
     padding: 0;
-    padding: 2px;
+    padding-left: 0.4em;
     width: 103px; }
   #conversejs #converse-roster .roster-filter.x {
     background-position: right 3px center; }

mockup/controlbox.html

@@ -126,9 +126,9 @@
                     <dd class="search-xmpp" style="display:none">
                         <ul>
                             <li>
-                                <form class="add-xmpp-contact">
+                                <form class="pure-form add-xmpp-contact">
                                     <input type="text" name="identifier" class="username" placeholder="Contact username">
-                                    <button type="submit">Add</button>
+                                    <button class="pure-button button-primary" type="submit">Add</button>
                                 </form>
                             </li>
                             <li></li>

mockup/index.html

@@ -87,9 +87,9 @@
                     <dd class="search-xmpp" style="display:none">
                         <ul>
                             <li>
-                                <form class="add-xmpp-contact">
+                                <form class="pure-form add-xmpp-contact">
                                     <input type="text" name="identifier" class="username" placeholder="Contact username">
-                                    <button type="submit">Add</button>
+                                    <button class="pure-button button-primary" type="submit">Add</button>
                                 </form>
                             </li>
                             <li></li>

sass/_controlbox.scss

@@ -330,7 +330,7 @@
 
         .add-xmpp-contact {
             background: none;
-            padding: 5px;
+            padding: 1em;
             input {
                 margin: 0 0 1rem;
                 width: 100%;

sass/_roster.scss

@@ -38,7 +38,7 @@
         height: $controlbox-dropdown-height;
         margin: 0 0 0.5em 7px;
         padding: 0;
-        padding: 2px;
+        padding-left: 0.4em;
         width: 103px;
     }
     /* (jQ addClass:) if input has value: */

src/templates/add_contact_form.html

 <li>
-    <form class="add-xmpp-contact">
+    <form class="pure-form add-xmpp-contact">
         <input type="text"
             name="identifier"
             class="username"
             placeholder="{{label_contact_username}}"/>
-        <button type="submit">{{label_add}}</button>
+        <button class="pure-button button-primary" type="submit">{{label_add}}</button>
     </form>
 </li>