I have been working on a theme that uses fusion (which I love), base and canvas as parents. I'm not a pro of CSS, but managed to get something done and decided to share it (as i found much useful info in the forums myself), just in case- and also ask a CSS pro who'd happen to read this if my solution is OK; I attached the relevant files in a zip-archive (can't share the whole theme right now).
I wanted to add a clickable logo top left of my theme, that would bring users back to the home page of our Moodle install, but no matter where I placed it, it moved other things around. In the end, I added a clickable logo with absolute position on a hidden link.
I based it all on Fusion for the layout: I first made changes in the general.php and frontpage.php (removing tagline, we don't use it, and moving the menus to the same height as login info) and added a div with a link with specific id in those files (I attached them in the zip file - there's a comment at the end so you can find the line easily) inside the page-header-wrapper div:
<div id="main-logodiv"><a href="<?php echo $CFG->wwwroot; ?>" title="<?php echo $GLOBALS['SITE']->fullname ?>" id="main-logo"><?php echo $GLOBALS['SITE']->fullname ?></a></div>
Then in my core.css, I added the following, make sure you edit the figures so that you position your logo exactly where you want, and that the sizes correspond to your logo pic, or it won't display properly; you can of course create different styles if your front page and other pages need a different logo..
height:75px; /* match width & height with #main-logo & main logo dimensions */
height:75px; /*make sure width & height match with the logo pic */
Hope this can be useful for sb else; any comments welcome!