Learning to modify theme/templates

Hi everyone,
Before I get started, if someone has a good reference for how to create a theme, I'd be very appreciative. The documentation here is a bit fragmented, and some examples would be hugely useful.

I'm getting started on modifying a Gallery2 theme (matrix) to match the Drupal theme I'm using. I've started with Greenhouse, and modified it for Drupal. That's fine and I'm pretty happy with that. Now I'm working on modifying the Gallery2 Matrix theme to match it. Some things I've been able to change without issue, but refinements are fighting back!

The page (for reference) is http://drupal.matinenda.ca/gallery2. One issue is that I'd like to "embellish" the 'Random Image' subheading with a background. But I can't get it to work. I was able to modify the block (change the bg, etc.) with this code:

div.block-imageblock-ImageBlock {
    padding: 5px 5px 5px 10px;
    margin: 10px 0px;
    background: #E1E9E2;
    border: 1px solid #468259;
    text-align: center;
    color: #468259;
}

I would then expect that this code would modify the 'Random Image' text:

div.block-imageblock-ImageBlock h3 {
    color: #FFFFF;
}

However, nothing happens.

Second
Say I wanted to add a title 'Links' to the core.ItemLinks block. Would the easiest way be to create a <div> around the {g->block type="core.ItemLinks" item=$child links=$child.itemLinks} call and put the formating and title there? ie

<div class="linksbox">
<h3> Links </h3>
{g->block type="core.ItemLinks" item=$child links=$child.itemLinks}
</div>

Is this the 'Gallery2 Way' to do it?