When using Adwaita-Dark, Epiphany has theming issues with some forms and in Private Browsing Mode.
When using Adwaita-Dark (and I'm guessing some other dark themes) in GNOME, the address bar on the Private Browsing mode is too dark to read and so are many web forms.
I'm guessing that the forms might be a problem with how GTK+ does the dark themes because Firefox also suffers from this problem (actually, Firefox is worse) unless you install an extension that provides a text contrast workaround![Webkitbugzilla], but maybe the Private Browsing theme could be tweaked a little to avoid the issue?
The first two images are a snippet of the forms over at Webkit's bugzilla, and how you have to highlight the text to get them to appear. The third is Private Browsing Mode with some text in the address bar.