If you are using firefox/chrome etc... you can find these codes by right clicking on the page and selecting "inspect element". You can see all the CSS codes in the right panel and they are generally named fairly logical. 

Also, most themes have options to changes these setting (admin/theme/settings) so I'd check there first.

Francisco, it is not that easy =)

Here is the deal.
general.html and dndindex.html files of your currently active theme assign certain styles to your whole website. For example the code bellow applies to whatever is under your header and menu:

<div class="ow_page_container">
            <div class="ow_menu_wrap">{$main_menu}</div>
            <div class="ow_canvas">
                <div class="ow_page ow_bg_color clearfix">
                    <h1 class="ow_stdmargin {$heading_icon_class}">{$heading}</h1>
                    <div class="ow_content">
                        {add_content key='base.add_page_top_content'}
                        {$content}
                        {add_content key='base.add_page_bottom_content'}
                    </div>
                    <div class="ow_sidebar">
                        {component class="BASE_CMP_Sidebar"}
                    </div>
                </div>
            </div>
        </div>
    </div>

Each of those <div>'s have their own styles. And so registration page, like all your other pages, uses those classes.

If you change the background in CSS for certain class, then changes will be applied sitewide.
Example (changing color ow the background for .ow_bg_color to black):



Sitewide effect:



My vision of how this can be done.

1. Add new class.
2. Assign this new class to the join page only ( in join_index.html).

But I am not a designer, may be other users have better solutions with detailed instructions.

Too complicated for me!

Topic was moved from General Questions.