app/views/devise/sessions/new.html.erb
<h2 class="devise-title">Log in</h2>
<div class="panel panel-default devise-panel col-md-4 col-sm-10 col-md-offset-4 col-sm-offset-1">
<%= form_for(resource, as: resource_name, url: session_path(resource_name)) do |f| %>
<div class="form-group">
<%= f.label :email %><br />
<%= f.email_field :email, autofocus: true, class: "form-control" %>
</div>
<div class="form-group">
<%= f.label :password %><br />
<%= f.password_field :password, autocomplete: "off", class: "form-control" %>
</div>
<% if devise_mapping.rememberable? -%>
<div class="form-group">
<%= f.check_box :remember_me %>
<%= f.label :remember_me %>
</div>
<% end -%>
<div class="actions">
<%= f.submit "Log in", class: "btn btn-primary form-control" %>
</div>
<% end %>
<%= render "devise/shared/links" %>
</div>