If the rendering is identical in both DEVONthink To Go and a standalone web browser, it is likely that the web page itself doesn’t properly support dark mode. That would be better fixed by the web site owner.
The website expects the web view to have a white background even in dark mode. This is a flaw in the web page design (they should set the background color instead of expecting a default), but we’ve fixed it for one of the next updates.