The reason your are seeing what you described is due to the content area having a semi-transparent image overlaying the background color on your body tag.
Here is the CSS code that is affecting your content area:
You could comment out the background on that style to remove the overlay or set the background-image property to none:
/* background-image: url("images/sheet.png"); -- commented out */
It would be best for you to establish a child theme if you are not already so that changes you make will not be lost if you update the theme in the future.