Hi there, @johnk. I'm here to help get this sorted out.

When activating the feature from the settings, all pages will turn into dark mode. Otherwise, if some pages remain bright white, I recommend clearing your browser's cache and cookies. Your browser stores these files to speed up websites. However, it can cause some problems in QuickBooks Online (QBO) when they become outdated. 

In the meantime, let's start by accessing your account through an incognito window to rule out the possibility of a webpage issue. You can refer to these shortcut keys to open an incognito window in all supported browsers:

• Chrome: Ctrl+Shift+N
• Firefox: Ctrl+Shift+P
• Safari: Command+Shift+N

Once logged in, make sure dark mode is turned on. If it works, I recommend that you clear its cache. By doing this, you can remove that historical data and access QuickBooks with a clean slate. 

I'm adding this link in case you need help in managing your account: Help Articles.

Visit me here again in the Community and keep me posted on your progress in getting this resolved. I'm determined to help you succeed.