feat(demo): add story 1 — Sorano: Rock and Time

plugins/problems/scss/mixins/_shadow.scss

@@ -0,0 +1,9 @@
+// Component focus shadow
+@mixin control-shadow($color: $primary-color) {
+  box-shadow: 0 0 0 .1rem rgba($color, .2);
+}
+
+// Shadow mixin
+@mixin shadow-variant($offset) {
+  box-shadow: 0 $offset ($offset + .05rem) * 2 rgba($dark-color, .3);
+}