変数の使い方
参考：
Set a variable in Sass depending on the selector – Stack Overflow
変数のスコープ
参考：
Understanding Variable Scope in Sass | Envato Tuts+
Should I be using global or local Sass variables? – Made by Munsters – Medium
Variable scoping in SASS – Stack Overflow
グローバル変数を書き換える
参考：
How to overwrite global variables in Sass. – GitHub Gist
How to update a global variable from mixin or function? · Issue #1511 · sass/sass – GitHub