Add sass pre-procesor:

yarn add node-sass sass-loader -D

Add bulma

yarn add bulma

Edit nuxt.config.js and add:

css: [
  { src: '~assets/sass/main.sass', lang: 'sass' }
]

Make this sass file on assets/sass/main.sass:

$primary: #4c4c6e
@import "../../node_modules/bulma/bulma.sass"

Finally run:

yarn run dev