⚒️ Restructured Sass organisation (#83)

*  🌈  Organise colors with vars only

*  ➕ New file structure

* File restructuring

*  ⚒️ Fix build issue

* Rearranged files

assets/css/abstracts/_universal-selector.scss

@@ -0,0 +1,11 @@
+* {
+  box-sizing: border-box;
+}
+
+*::before {
+  box-sizing: border-box;
+}
+
+*::after {
+  box-sizing: border-box;
+}