How to apply css Styling in Ruby on Rails HTML page?

I am new in RoR. I am trying to add css classes on my form. Here is my form:

<%= form_tag :action => 'create' do %>
  <p><label for = "name">Name</label>:
    <%= text_field 'books', 'name' %></p>
  <div class="form-control">
  <p><label for = "author">Author</label>:
    <%= text_field 'books', 'author'%></p>
  </div>
  <p><label for = "price">Price</label><br/>
    <%= text_area 'books', 'price'%></p>
  <%= submit_tag "Create" %>
<% end -%>

That form controls does not accepting class="form-control mb-4 col-10" placeholder="Patient name" like <input type="text" th:field="*{name}" class="form-control mb-4 col-10" placeholder="Patient name">. How can I apply css styling.