Change Border Property By ID

I am working on creating a simple image gallery. I have added styling around the thumbnail images (border and padding). I would like to change the border color when the item is hovered over. I know this can be done rather easily with the getElementById command. Can someone help me understand this process? Here's a snippet of the list that I am working with...

Here's the CSS:

ul {
padding:0;
margin:0;
position:relative;
}
li {
float:left;
padding:0 .5em .3em .5em;
list-style:none;
}
#gallery {
list-style:none;
}
#gallery li {
display:inline;
}
#gallery li a img {
border:1px solid #ccc;
padding:3px;
}

Here's the HTML:

<ul id="gallery">
<li><a href="path/to/image/large/" title="title/of/image/"><img src="path/to/image/thumb/" alt="title/of/image/" /></a></li>
<li><a href="path/to/image/large/" title="title/of/image/"><img src="path/to/image/thumb/" alt="title/of/image/" /></a></li>
<li><a href="path/to/image/large/" title="title/of/image/"><img src="path/to/image/thumb/" alt="title/of/image/" /></a></li>
</ul>
I would like to change the border color from #ccc to #3455ef when the item is hovered over, and ideally have a different style when the item is clicked (or active).