New CSS Theme / Convert ugly table-based website to HTML5/CSS

We have a website: [login to view URL] We also have a hotspot portal: [login to view URL] We need a simple, modular theme - basically just CSS, with some HTML templates, that we can wrap everything into. With some neat widgets we can use. A theme that will scale well to all devices. We're open to something like JQuery, but want the lightest weight possible, and the widest compatibility possible. The back-end is all our own code, which we don't mind sharing. It's simple really, currently there are four directories, one for headers, footers, sidebar and body. The [login to view URL] file simply holds an array of which files in each folder to show for which page. It's quite modular - the menu, for example, automatically lists all the pages.. But you needn't worry about this - the most important is that we get rid of our current table-driven website that looks ugly on mobile devices.