fix: unquoted custom properties

Author: mvsde

lib/index.js

@@ -251,7 +251,7 @@ module.exports = class Breakpoints {
     if (this.#config.css.customProperty) {
       string += `${this.#config.css.element} {`;
       this.#customProperties.forEach((customProperty) => {
-        string += `--${customProperty.name}: "${customProperty.value}";`;
+        string += `--${customProperty.name}: ${customProperty.value};`;
       });
       string += `}`;
     }