Github didn't "go" light text on black background. You chose their dark theme at some point after it was offered. The default is light.
And no... Sigil's new website will not offer a similar user choice. Though I may choose to change the theme's skin from time to time.
Besides... no one's forcing you to use the new website. You're free to use github's dark-theme, log-first, traditional Sigil release pages for just as long as you like.
Out of curiosity: did you ever utilize the old Wordpress sigil-ebook.com website this new one replaced?