css – Bootstrap styles conflicts with WordPress theme

I have an existing WP theme (not mine), and I need to implement some custom pages, lets say that I want to create a calculator on the custom page (the theme layout is the same as everywhere else, but there is calculator on one of the website pages). For that, I need to use Bootstrap. However, the theme that I use does not include it, so I need to add it. I added it this way:

function addBootstrapCss() {
  wp_enqueue_style('bootstrap-css-scripts', get_template_directory_uri() . '/assets/css/bootstrap.min.css', (), 1);
add_action('wp_enqueue_scripts', 'addBootstrapCss' );

Easy one. However, this Bootstrap conflicts with the theme (theme design is broken – backgrounds, buttons, etc.). Can somebody advise how to solve this? I need to keep the theme intact and I need to use it on my custom pages (on those pages I have to keep the theme layout and design, of course).