Magento 2 default bootstrap not applied on custom theme

Question

I am using magento 2.4.1 open source, i am using custom theme inherit from blank theme, for home page design i have created static block and insert as widgets in CMS page as my home page. I am using bootstrap design on my home page but its not working, after added bootstrap cdn to my

\app\design\frontend\zero_theme\mytheme\Magento_Theme\layout\default_head_blocks.xml

<css src="https://cdn.jsdelivr.net/npm/bootstrap@4.5.3/dist/css/bootstrap.min.css"  src_type="url" rel="stylesheet" type="text/css" /> 

then itsworking, magento have default bootstrap right, why its working.

Note : After declare bootstrap my PLP & PDP some of the alignment collapsed.

How to handle bootstrap in custom theme?