The HTML & CSS you include in "HTML Box" will be sandboxed, so you cannot use it to apply styles to elements in the rest of the page, unfortunately.
I got this to work on my site with the following code
<style type="text/css">
#tester {
padding: 100px;
height: 500px;
width: 500px;
border: 1px solid #0F0;
background-color: #ff0000;
}
</style>
<div id="tester">please work 0_0</div>
By default it seems that when you're in editor mode, none of the custom CSS in the HTML box is applied. If you go to the top right corner of the page, click MORE > Preview Page as Viewer it will display correctly.