You could use jQuery's class management methods (namely addClass() and removeClass() in this case) to add a class on the selected item and remove the same class from all the other items (if you want only one selected at a time).

//save class name so it can be reused easily

//if I want to change it, I have to change it one place

var classHighlight = 'highlight'; 



//.click() will return the result of $('.thumbnail')

//I save it for future reference so I don't have to query the DOM again

var $thumbs = $('.thumbnail').click(function(e) {

    e.preventDefault();

    //run removeClass on every element

    //if the elements are not static, you might want to rerun $('.thumbnail')

    //instead of the saved $thumbs

    $thumbs.removeClass(classHighlight);

    //add the class to the currently clicked element (this)

    $(this).addClass(classHighlight);

});

Then in your CSS just add:

.highlight {

    background-color: cyan;

    font-weight: bold;

}

jsFiddle Demo

This is a better solution than changing CSS properties directly from jQuery/Javascript (with the .css() method for example), because separation of concerns will make your code more manageable and readable.