[docs-cashtab] night mode default and twitter preview card
Summary: Modify themeConfig settings to set dark mode to default and add a twitter preview banner.
Test Plan: npm start and observe loads in dark mode. Twitter preview card will need to be tested after deployment.
Reviewers: #bitcoin_abc, Mengerian
Reviewed By: #bitcoin_abc, Mengerian
Differential Revision: https://reviews.bitcoinabc.org/D9203