With banner images, there is an option to change the colour of the overlay and to also change the transparency level. The default colour is black and the transparency is set to be just dark enough to allow any text to show up.
- Go to the Appearance > Customize > Theme Options > Other Options
- Look for the Cover Overlay settings and use the colour selector to change the colour:
- Use the slider to adjust the overlay transparency
- Click the Publish button.