You can specify the background-color in the wp-content/themes/[theme name]/style.css file.
If you look in the theme's header.php file, you should see something like
PHP Code:
<div id="header">
<h1 id="blog-title"><a href="<?php echo get_option('home') ?>/" title="<?php bloginfo('name') ?>" rel="home"><?php bloginfo('name') ?></a></h1>
<div id="blog-description"><?php bloginfo('description') ?></div>
</div><!-- #header -->
This is what you need to change. Just make sure you provide a text alternative to the image for those with no images enabled. And I'm not sure how this would affect the SEO, as bots read text, not images.
As can be seen by looking at the wplancer site, the resulting mark-up is
HTML Code:
<div id="header">
<div id="logo">
<h1>
<a href="http://www.wplancer.com">
<img src="http://www.wplancer.com/wp-content/themes/wplancer/img/logo.png" alt="WordPress Freelancer" border="0" />
<span>WordPress Freelancer</span>
</a>
</h1>
</div>
.....
The CSS has
Code CSS:
#header{
overflow: hidden;
}
#logo{
float: left;
}
H1 {
margin: 0;
padding: 0;
}
H1 span{
font-size: 10px;
display: none;
float: left;
margin: 0;
padding: 0;
}
so that the text is still there and will show if images is off.
BTW. the wplancer site has been compromised. If you downloaded your theme from that site you should run an AV/trojan/spyware scan ASAP.
Bookmarks