Use escape-svg() function (#29077)

* Use escape-svg() function

* Update theming.md

scss/_functions.scss

@@ -71,6 +71,17 @@
   @return $string;
 }
 
+// See https://codepen.io/kevinweber/pen/dXWoRw
+@function escape-svg($string) {
+  @if str-index($string, "data:image/svg+xml") {
+    @each $char, $encoded in $escaped-characters {
+      $string: str-replace($string, $char, $encoded);
+    }
+  }
+
+  @return $string;
+}
+
 // Color contrast
 @function color-yiq($color, $dark: $yiq-text-dark, $light: $yiq-text-light) {
   $r: red($color);