Add light theme first pass

Toggle is only temp to test. Need to move in nav for final.
Will need further adjusting colors.

styles/light/_mixins.scss

@@ -0,0 +1,5 @@
+@mixin prefers-color-scheme( $scheme: dark ) {
+    @media (prefers-color-scheme: #{$scheme}) {
+      @content;
+    }
+  }