Update customizer

Add all the new things to customizer:
* `@headings-font-family`
* `@container-*`
* darker `@btn-default-bg`

docs/customize.html

@@ -398,9 +398,20 @@ lead: Customize Bootstrap's components, LESS variables, and jQuery plugins to ge
           <label>@line-height-base</label>
           <input type="text" placeholder="1.428571429">
           <p class="help-block">Used to calculate line-height throughout Bootstrap.</p>
+        </div>
+      </div>
+
+      <h4>Headings</h4>
+      <div class="row">
+        <div class="col-lg-6">
+          <label>@headings-font-family</label>
+          <input type="text" placeholder="@font-family-base">
+          <p class="help-block">Choose a separate font-family for headings.</p>
           <label>@headings-font-weight</label>
           <input type="text" placeholder="500">
           <p class="help-block">Choose a separate font-weight for headings.</p>
+        </div>
+        <div class="col-lg-6">
           <label>@headings-line-height</label>
           <input type="text" placeholder="1.1">
           <p class="help-block">Choose a separate line-height for headings.</p>
@@ -458,6 +469,24 @@ lead: Customize Bootstrap's components, LESS variables, and jQuery plugins to ge
         </div>
       </div>
 
+      <h3>Container sizes</h3>
+      <p>Define the maximum width of <code>.container</code> for different screen sizes.</p>
+      <div class="row">
+        <div class="col-lg-6">
+          <label>@container-tablet</label>
+          <input type="text" placeholder="728px">
+          <p class="help-block">For <code>@screen-small</code> and up.</p>
+          <label>@container-desktop</label>
+          <input type="text" placeholder="940px">
+          <p class="help-block">For <code>@screen-medium</code> and up.</p>
+        </div>
+        <div class="col-lg-6">
+          <label>@container-large-desktop</label>
+          <input type="text" placeholder="1170px">
+          <p class="help-block">For <code>@screen-large</code> and up.</p>
+        </div>
+      </div>
+
       <h3>Grid system</h3>
       <p>Define your custom responsive grid.</p>
       <div class="row">
@@ -522,7 +551,7 @@ lead: Customize Bootstrap's components, LESS variables, and jQuery plugins to ge
           <label>@btn-default-color</label>
           <input type="text" placeholder="#fff">
           <label>@btn-default-bg</label>
-          <input type="text" placeholder="#a7a99aa">
+          <input type="text" placeholder="#474949">
           <label>@btn-default-border</label>
           <input type="text" placeholder="@btn-default-bg">
           <h4>Primary</h4>