How can I add a theme template to site?
Follow the following steps to add the bootstrap:

1. Put the bootstrap theme in the theme folder.
2. If you want to create a sub theme then copy the cdn folder from starterkits and rename. (themename)
3. Rename .info.yml , .libraries.yml and .theme files with your themename. 


Include the CSS in theme.libraries.yml file. Like below

global-styling:
 css:
  theme:
    css/style.css: {}

global-scripting:
 css:
  theme:
   js/script.js: {}