How to load Bootstrap by code

I dont want Bootstrap always load when my app running. I want when i clicking a button then Bootstrap.css will start loading.

How to do that ?
Thanks.

Thank you intregal, how can i show a html file in a view:‘window’ but css of this html won’t affect to my app ?

try to use iframe component
https://docs.webix.com/api__refs__ui.iframe.html

do you have anyway to show in iframe the code below:
“hello”

i mean a string with html, head and body tag.