DT3’s built-in web browser has dark background in dark mode. It appears some sites do not set white background and instead rely on the browsers default white background. This makes some site render black text on dark background which makes it very hard to read.
That is under the control of the sites but you can try disabling Preferences > General > Appearance > Use dark backgrounds for documents
I understand that this is controlled by the websites. However all (major?) browsers set window background to white even in dark mode. Websites look OK in all browsers even when they keep default background transparent.
I suggest setting window background to white for websites. If the engine allows expose current theme (light/dark) and let the site decide if it renders in dark mode. Current web standards have this covered. All major browsers support this feature too.
This will fix websites and still let people have dark background for other documents.