how do i use Bulma variables in my own Sass?

This answer says to @import 'path/to/bulma/sass/utilities/initial-variables';

which will … work, but not if i change a variable.

A brief aside on how Bulma initial variables work: They are all defined with !default at the end, which tells Sass to only use the value provided if the variable doesn’t already exist. That’s how you can override Bulma variables before you load Bulma.