You can find the final css build at projectName/node_modules/bulma/css/bulma.css.
Chances are you’re using a file loader with webpack and similar. If, for example in a Vue project, you have that, then you can use import syntax:
import 'bulma/css/bulma.css'
within your js. This works because having import [xyz from] 'xyz' will look at projectName/node_modules/xyz, and in the case of a css file, it’s as simple as that!
If you do not have that installed, you need to find a way to send it over to the client. Just copy projectName/node_modules/bulma/css/bulma.css into a file, maybe bulma.css, in either an assets or public or whatever you use, then fetch it like you’d fetch any css file within the html: <link rel="stylesheet" href="https://stackoverflow.com/bulma.css">