feat(isabelle-edits): Clarify theme structure and add SVG icon support

Improves theme organization by adopting a 7-in-1 SCSS structure.

Introduces support for SVG icons within templates, enhancing visual flexibility.

Adds a checkmark SVG asset.

falcon/_dev/css/theme/custom/abstracts/_utilities.scss

@@ -1,3 +1,7 @@
+// Header selector for interpolation
+// Example usage: #{$headings} { ... }
+$headings: "h1, h2, h3, h4, h5, h6, .h1, .h2, .h3, .h4, .h5, .h6";
+
 // Color utility classes
 @each $name, $color in $theme-colors {
     .bg-#{$name} {