White text color appears normally because the background color is dark.
It looks nice on the front of the pages but we can't see in the backend of the site because the white text is blending with the text background.
To edit that you must change the text color in the backend site to black.
- Highlight all the text
- Click "Toolbar Toggle" and then "Text color" pick black color.
- Here is the appearing before and after.
- After you finish edit your contents, don't forget to change it back to white.