Changing page layout for Contribution pages in Joomla
In Joomla, it's actually not too difficult to override the CSS of any particular page. You can give your page a custom CSS class, then append the CSS to the end of your template CSS file, or a custom CSS file that is pulled into the page.


If you don't already have one, create a menu item for the particular Contributions pages you want to override. You can create a hidden or links menu in Joomla that doesn't have an associated module. Select the CiviCRM - Contribution page type, then the page. You'll need to do this separately for each page.
Under Page Display Options for the menu item, add the new class into the Page Class box.


A few notes: When I create CSS overrides for individual pages, I usually define the page class, then use Firebug (or something similar) to test the overrides prior to uploading them. Depending on how and when the CSS of your template loads, you may need to make use of the !important declaration. 

Also, the use of display:none usually works for me in the case of having to hide certain elements, but there is some debate about when you should and shouldn't use that. 

Another option that's a little more work than creating a template override is to create an entirely new template. One of the features that Joomla has over certain other CMSes is the ease with which you can assign different templates to individual pages via the template manager. This would completely avoid having to hide certain elements, because you would simply leave them out of your new template. You could start with your current template, and just remove the pieces that you don't need. Install it via the Extension manager, and assign it to your Contribution pages. Note that you will still need to create a menu item for each Contribution page in question.