If the text and background colors of your content blocks don't match your theme when you put them in edit mode, you may need to modify your theme's typography.css file. This file is also used as the theme for tinyMCE-- the content block editor.

For example, if you define

body {
  color: white;
} 

your body text will be white. But if you neglect to define a background color, the background in tinyMCE would be white as well-- making it impossible to see the text you're trying to edit. Make sure to define the background color of your body in the typography as well.