yes, you can surely do this. Go to your project directory, if you are using windows your project will be located in the Documents/Mendix folder by default.
Yes, look at the documentation on Customizing Styling, in particular, section 2.4 as it gives an example of bringing in a third-party library.
https://docs.mendix.com/howto/front-end/customize-styling-new/#24-importing-css-web-only
Mendix already has bootstrap available so you shouldn’t need to import this again.
https://docs.mendix.com/howto/front-end/styles/
Hope this helps.
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