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First, download a stand-alone development server like XSAS or easyPHP. Get that all setup and install a fresh copy of XOOPS 2.0.7 in a directory called Beta, or whatever. Just not in the root because you may want to tinker with more than one site at a time.
Next, browse to x:\xoopsroot\themes\default and open your theme.html document in Dreamweaver in either code or split view.
The following code is your header:
<table cellspacing="0">
<tr id="header">
<td id="headerlogo"><a href="<{$xoops_url}>/"><img src="<{$xoops_imageurl}>logo.gif" width="150" height="80" alt="" />a>td>
<td id="headerbanner"><{$xoops_banner}>td>
tr>
<tr>
<td id="headerbar" colspan="2"> td>
tr>
table>
Now, start tinkering. Just remember to change the logo width and height if you're going to use a different logo, otherwise it can get real ugly. Also, with each new save, you'll need to empty the templates_c directory in order to view the changes you've made.
Tinker around with the default theme for a little bit so you can get comfortable with how it's done, then install the theme you want to edit and hack away! Every theme is different and some have some pretty ugly code, but if you get the basics down first, you'll be able to spot the rest later.
Best of luck!