When we download the WordPress for our website then by default it has some themes. Which is not have any bootstrap files.
so if we want to add bootstrap in WordPress theme then it has 3 way to add the bootstrap file in our WordPress theme-
1. Adding Bootstrap through Function.php file
2. Link to Bootstrap CDN

Method 1:- Adding Bootstrap through Function.php file
STEP 1:- Before we get started, there are a few things we need to do-

Install WordPress
Download and Unzip Bootstrap
Step 2:- Once you have these things ready, open the directory with all your WordPress files and navigate to

wp-content\themes\twentyseventeen\

Step 3:- Once you navigate to that folder Inside of that folder paste the bootstrap folder which we unzip before.

Step 4:- Now open your ” Function.php ” file and copy below code after the ” register_sidebar ” function.

/…………………………..Copy From Here………………………………………………………./

function theme_styles() 
{
    wp_enqueue_style( 'bootstrap_css', get_template_directory_uri() . '/css/bootstrap.css' );
    wp_enqueue_style( 'bootstrap_responsive_css', get_template_directory_uri() . '/css/bootstrap-responsive.css' );
}
add_action( 'wp_enqueue_scripts', 'theme_styles');

function theme_js() 
{
    global $wp_scripts;
    wp_enqueue_script( 'bootstrap_js', get_template_directory_uri() . '/js/bootstrap.min.js' );
}
add_action( 'wp_enqueue_scripts', 'theme_js');

/…………………………..Copy To Here………………………………………………………./

Note:- you can paste this code anywhere in ” function.php ” file but i paste it under the ” register_sidebar ” function.
Now your code look like it

 'before_title'  => '',
    'after_title'   => '',
) );
}
add_action( 'widgets_init', 'twentyseventeen_widgets_init' );

function theme_styles() 
{
    wp_enqueue_style( 'bootstrap_css', get_template_directory_uri() . '/css/bootstrap.css' );
    wp_enqueue_style( 'bootstrap_responsive_css', get_template_directory_uri() . '/css/bootstrap-responsive.css' );
}
add_action( 'wp_enqueue_scripts', 'theme_styles');

function theme_js() 
{
    global $wp_scripts;
    wp_enqueue_script( 'bootstrap_js', get_template_directory_uri() . '/js/bootstrap.min.js' );
}
add_action( 'wp_enqueue_scripts', 'theme_js');

/*................................Copy To Here................................................................*/

/**
 * Replaces "[...]" (appended to automatically generated excerpts) with ... and
 * a 'Continue reading' link.

Step 5:- Now save the “Function.php” file. That’s it now your wordpress theme is fully responsive

Method 2:-Link to Bootstrap CDN
STEP 1:- open your ” Function.php ” file and copy below code after the ” register_sidebar ” function.

/…………………………..Copy From Here………………………………………………………./

function theme_styles() 
{
    wp_enqueue_style( 'bootstrap_css', 'https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap-theme.min.css' );
}
add_action( 'wp_enqueue_scripts', 'theme_styles');

function theme_js() 
{
    global $wp_scripts;
    wp_enqueue_script( 'bootstrap_js', 'https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js');
}
add_action( 'wp_enqueue_scripts', 'theme_js');

…………………………..Copy To Here………………………………………………………./

Note:- you can paste this code anywhere in ” function.php ” file but i paste it under the ” register_sidebar ” function.
Now your code look like it

      'before_title'  => '',
        'after_title'   => '',
    ) );
}
add_action( 'widgets_init', 'twentyseventeen_widgets_init' );

/*................................Copy From Here................................................................*/

function theme_styles() 
{
    wp_enqueue_style( 'bootstrap_css', 'https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap-theme.min.css' );
}
add_action( 'wp_enqueue_scripts', 'theme_styles');

function theme_js() 
{
    global $wp_scripts;
    wp_enqueue_script( 'bootstrap_js', 'https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js');
}
add_action( 'wp_enqueue_scripts', 'theme_js');

/*................................Copy To Here................................................................*/

/**
 * Replaces "[...]" (appended to automatically generated excerpts) with ... and
 * a 'Continue reading' link.

Step 2:- Now save the “Function.php” file. That’s it now your wordpress theme is fully responsive