Add nord theme with license header, update all theme files with license

gruvbox.css

@@ -1,4 +1,9 @@
-/* Gruvbox color theme - alternative to Ash */
+/* Gruvbox color theme by Pavel Pertsev
+ * https://github.com/gruvbox-community/gruvbox
+ * License: BSD-3-Clause
+ *
+ * Each color has light/dark variants (50% interpolation).
+   Variant names follow the pattern: base, base-light, base-dark. */
 
 :root {
     /* Base surfaces (dark) */