Well there's a lot here, but I can give you a couple of tips.

First, avoid repeating yourself in the CSS. For example, in your web.css sheet you tell the font to be Trebuchet MS 8 times. Define your main font once on the body tag, and then only define it again on elements that should have a different font. The same goes for the rest of your CSS, try to define things only once. You should only add things that are different.

Second, it's good practice to start with a core stylesheet that applies to everything. You might think that's your web.css, but that only goes up to 1336px. On my large widescreen monitor (1920px) your site has no styles at all. When writing your core.css it's a good idea to start with the styles for the smallest mobile view. Then in your other media queries, add the styles for the larger sizes. Once again, don't repeat yourself. Only add styles that should be different for tablet or desktop.