Hi, and thanks for asking.
Don't worry about layout, it is not part of the judging process. The ideal format for me is a plain text document. That said, you might want to design your Spark the way you want to, to give it impact and appeal, if you want potential players to download it.
The previous Sparks were fantasy but I encourage and welcome more settings. I'm not sure if you know this but the next CoinSides will be CyberPunk themed: https://www.kickstarter.com/projects/ex1stgames/cyber-coinsides-10-dice-and-more-on-one-spinning-coin
Linking Sparks is a great idea, proving they are optional and work well as stand-alone Sparks.
I hope this answers your questions.