Solved.

https://github.com/quasarframework/quasar/issues/8457

It should be written as:

<style lang="sass">
.q-toolbar__title
  font-size: 30px
  @media (max-width: $breakpoint-xs-max)
    text-align: center
.q-footer
  .q-tab__icon
    font-size: 30px
</style>