Follow the Material Design color guidelines
It appears to me that F-Droid uses full black #000000 color on white background.
The Material Design color guidlines state that primary text should use 87% opacity.
Please follow the Material Design color guidelines: https://www.google.com/design/spec/style/color.html
More specially the UI color application section: https://www.google.com/design/spec/style/color.html#color-ui-color-application