First of all, it depends on what theme you have. Many premium themes provide an option for users to enter their custom CSS. It that is the case you can add your styles there.
If not, follow the link to create a child theme – http://www.wpbeginner.com/wp-themes/how-to-create-a-wordpress-child-theme-video/
You can add your custom style in child theme’s style.css.
For HTML files. you will need to create template files in your child theme. First, you need to know which page uses which template file. Read the link for that – https://developer.wordpress.org/themes/basics/template-hierarchy/
Suppose you need to edit the single blog page and add some HTML code to it. You will need to create your own single.php in your child theme. You can copy the single.php from parent theme and add your own HTML to it. Save the file in your child theme folder and it will override the parent theme’s template file.
Hope this helps.