<div id='modal-register' class='modal'>
  <div class='modal-content'>
    <span class='close'>x</span>

      <div class="pure-g">

      <div class="pure-u-1-3">
        <% if flash[:user_create_error] %>
          <div class="error"><%= flash[:user_create_error] %></div>
        <% end %>
        <% if flash[:user_successful_destroy] %>
          <div class="error"><%= flash[:user_successful_destroy] %></div>
        <% end %>
      </div>
      <div class="pure-u-1-3 user-form">
        <h2>Register</h2>
        <%= form_for :user, url: users_path, html: { class: "pure-form pure-form-stacked"} do |f| %>
          <div class="pure-control-group">
            <%= f.label :username %>
            <%= f.text_field :username, placeholder: "Username" %>
          </div>
          <div class="pure-control-group">
            <%= f.label :email %>
            <%= f.email_field :email, placeholder: "Email" %>

          </div>
          <div class="pure-control-group">
            <%= f.label :password %>
            <%= f.password_field :password, placeholder: "Password" %>
          </div>
          <div class="pure-control-group">
            <%= f.submit :Register, class: 'pure-button pure-button-primary' %>
          </div>
        <% end %>
  </div>
</div>

  </div>
</div>

<div id='modal-register' class='modal'>
  <div class='modal-content'>
    <span class='close'>x</span>

      <div class="pure-g">

      <div class="pure-u-1">
        <% if flash[:user_create_error] %>
          <div class="error"><%= flash[:user_create_error] %></div>
        <% end %>
        <% if flash[:user_successful_destroy] %>
          <div class="error"><%= flash[:user_successful_destroy] %></div>
        <% end %>
      </div>
      <div class="pure-u-1 user-form">
        <h2>Register</h2>
        <%= form_for :user, url: users_path, html: { class: "pure-form pure-form-stacked"} do |f| %>
          <div class="pure-control-group">
            <%= f.label :username %>
            <%= f.text_field :username, placeholder: "Username" %>
          </div>
          <div class="pure-control-group">
            <%= f.label :email %>
            <%= f.email_field :email, placeholder: "Email" %>

          </div>
          <div class="pure-control-group">
            <%= f.label :password %>
            <%= f.password_field :password, placeholder: "Password" %>
          </div>
          <div class="pure-control-group">
            <%= f.submit :Register, class: 'pure-button pure-button-primary' %>
          </div>
        <% end %>
  </div>
</div>

  </div>
</div>

Update pure grid to u-1.
Update pure grid to u-1.
