Create a spooky custom theme for PF demo

[Figma assets](https://www.figma.com/design/wLv12GzmtBov2lgEsbgZOj/Spooky-PatternFly%3A-Design-Tokens-%26-Styles?node-id=8402-2&node-type=canvas&t=9m2ie4scEiZjeyTS-0) provided by the talented @lboehling 

[Design tokens PR](https://github.com/patternfly/design-tokens/pull/95) from the master of disaster @srambach 