Welcome to Meteor and to the forum!
The twbs:boostrap package is outdated and you should use the npm packages instead. To install bootstrap@latest you can cd into your Meteor project root and enter the following into your terminal:
$ meteor npm install --save bootstrap@latest jquery@latest popper.js@latest
Then import it on the client on some top-level file:
import 'bootstrap'
import popper from 'popper.js'
// global.Popper = popper // uncomment when modals etc. are not working properly
To install a certain bootstrap theme, you need to include the unminified css or scss files in your project and import the files like any js / jsx file.