How can you change the color of a link within Google Sites when the mouse hovers over the link?
Although I have not used Google Sites, I believe the problem you are having is that it does not let you add custom CSS?
Having checked Google and finding a "CSS Workaround" I see that while you cannot add custom CSS, you can add custom HTML. Although most people prefer using CSS for styling, you can still add CSS to the attributes in HTML.
Unfortunately, HTML Box content is displayed on page with delay