@aukfood Thanks for the screenshot.
That looks like your Font Color is accidentally (forced) set to “Black”.
If you:
- Highlight the cells (or press Ctrl+A to highlight everything)
- Click on the little “Font Color” button.
- Press “Automatic” button.
That should fix your problem. 
Here’s an image showing where it’s located:
And here’s where you can see the difference in Dark Mode:
- A1 has “Black” color text.
- A2 has “Automatic” color text.
Side Note: There are also plans to enhance Dark Mode in the coming versions:
There will be a new button to allow you to “invert the background color” of the document as well. So you’ll pretty much have 2 separate toggles:
- Change UI to Light/Dark
- Change document’s background from Light/Dark
This will help in situations like this, where the person who created the document accidentally FORCED certain text colors.