If you post a link to your site, it would be easier to offer suggestions.
Also, I would suggest learning how to use a web debugging tool like Firebug (a free extension for Firefox) or Chrome Developer Tools, which comes built-in with Chrome. Those tools will help you see what CSS rules are being applied to different elements on the page, and you can interactively make changes before deciding on your final CSS.
Unfortunately, the Spun theme doesn't have a custom CSS option, so if you have not yet created a child theme, then I would use a custom CSS plugin like Custom CSS Manager. Otherwise, you can add your custom CSS to the end of your child theme's style.css file.
Assuming that you set the background image of the body element, the main elements that you want to alter are the page DIV (which holds the header, the main content area, and the footer) and the hentry DIV (holding the post/page content, contained several levels deep within the page DIV). So play around with the following combination of CSS rules:
background-color: rgba(185, 185, 185, 0.7);
background-color: rgba(250, 250, 250, 0.7);
You can use either or both rules for page and hentry. The parameters for the rgba function are red, green, blue, and opacity. The color values range from 0 to 255, with white being 255, 255, 255 and black being 0, 0, 0. This color picker will help you pick the correct values for the color/transparency that you want.
Lastly, other questions about the Spun theme can be posted on their support page.