Select the "Menu" (hamburger) icon at the top left, then click on "Sign in".
In the sign in page, both the "Email" and the "Password" fields are displayed with white foreground text on white background, i.e. invisible
p.s. this is with Fedora 26, default Wayland login, and the included adwaita-dark theme
Firefox 54.0.1
I can reproduce. It looks like one of the iframe edge cases. I think I know what the issue is; may take me some time to figure out a clean way to fix it.
In the popular mapping site "HERE", when selecting "Sign in":
From: https://wego.here.com
Select the "Menu" (hamburger) icon at the top left, then click on "Sign in". In the sign in page, both the "Email" and the "Password" fields are displayed with white foreground text on white background, i.e. invisible
p.s. this is with Fedora 26, default Wayland login, and the included adwaita-dark theme Firefox 54.0.1