[e.cash] Add styles to staking warning box

Summary: Adding a bit of styles to the warning paragraph on the staking page to make it stand out more

Test Plan: preview the site and check out /staking. See if the the warning box looks good

Reviewers: #bitcoin_abc, Mengerian

Reviewed By: #bitcoin_abc, Mengerian

Subscribers: Mengerian

Differential Revision: https://reviews.bitcoinabc.org/D14773