Skipping through the technical jargon as much as possible, If you look at the source, toward the top, between the head tags, you should see links to css files. Somewhere in those files will be where the background hexadecimal color is specified.
That color will be part of a class that’s likely associated with a container div in the HTML
In any case, you’ll need to make the change in the CSS file once you locate the problem. The same general principle is true of tracking down the issue with blue line.
If you’re using Chrome, instead of looking at View Source, look at the Developer Tools item associated with it. There you will see what is, essentially, the source HTML file with the ability to experiment with various parameters associated with the page. Clicking the different page elements will cause their associated CSS classes and various attributes (including hex values) to be displayed beneath the source code in a separate box.
You can actually change those values to see what effect doing so has on the page. This makes a good way to locate a problem before heading into the server files, tracking down the CSS and making the permanent fix.
Sounds complicated, I know. If you want to send me a link to the actual page, I can open the developer tools and try to track down the location problem myself (no charge). I don’t need access to the server or anything like that. I’m not familiar with the Woo Theme, but there’s also the possibility of it using a stylesheet technology known as LESS instead of straight CSS. If that or something similar is going on, it gets more complicated.