feat(main): Add base theme: This is the falcon theme out of the box.

This is falcon v3.1.2

_dev/css/abstracts/functions/_index.scss

@@ -0,0 +1 @@
+@import "rem-calc";

_dev/css/abstracts/functions/_rem-calc.scss

@@ -0,0 +1,5 @@
+@use "sass:math";
+
+@function rem-calc($size) {
+  @return math.div($size, 16px) * 1rem;
+}