app/views/devise/sessions/new.html.erb
<div class="panel panel-primary">
<div class="panel-heading">
<h3 class="panel-title">Sign In</h3>
</div>
<div class="panel-body">
<%= form_for(resource, :as => resource_name, :url => session_path(resource_name),html: {class: 'form-horizontal'}) do |f| %>
<div class="form-group">
<%= f.label :email, class: "col-sm-3 control-label" %>
<div class="col-sm-5">
<%= f.email_field :email, class: "form-control" %>
</div>
</div>
<div class="form-group">
<%= f.label :password, class: "col-sm-3 control-label" %>
<div class="col-sm-5">
<%= f.password_field :password, autocomplete: "off", class: "form-control" %>
</div>
</div>
<div class="form-group">
<div class="col-sm-offset-3 col-sm-5">
<%= f.submit "Sign In", class: "btn btn-primary" %>
</div>
</div>
<% end %>
</div>