To include an account login form in your theme, you just need to create an HTML form with its action attribute set to /account/login
Within this form you should have an input of type text with a name attribute set to to email and another attribute set to to password.
An example is shown below.
<form action="/account/login" method="post"> <input type="text" name="email"> <input type="text" name="password"> <button type="submit">login</button> </form>
Once the login form has been submitted the user is redirected to the account/home view.
You can append the string ?login_redirect=X to the end of the action path in order to specify if you would like a user to be redirected to another page once they have logged in. Where X is the full URL of the page you would like the user to be redirected to.