If you throughly studied the very many settings of the Photo Albums -> Settings admin page and you still can not get the display you want, there are two ways to make your customized CSS.
Method one: Add your wppa+ related css changes in Table IV-A15. This is far most the easyest way to customize your CSS. It is update safe and easy to maintain/alter.
Method two: Modify the wppa+ style sheet wppa-style.css. This method is strongly discouraged, but is still maintained for backward compatibility.
To avoid overwriting your modifications when an upgrade of WPPA+ occurs (to make it update-proof), you should make a copy of .../wordpress/wp-content/plugins/wp-photo-album-plus/theme/wppa-style.css and place it in your Theme directory (for instance: .../wordpress/wp-content/themes/twentyten/ ) and edit that file to make your changes.
To use this file, you have to tick the box in Table IV-A16 first. Now, as soon as the plugin finds a wppa-style.css file in your active theme directory, it will load that one and ignore the original one. This also means that, if you regret your alterations, just delete the copy in the theme directory, and things are back to original.
By modifying this Stylesheet, be aware of the fact that the settings in the Settings admin page have priority over what you put in the css file. So, if you want to set a background color in the css file, make sure the corresponding setting in the Settings screen is left (or made) blank, or add !important to your css specidication.
Also, be aware of the fact that a new version of the plugin may imply changes in the 'official' css file that are vital for the proper functioning of new features of WPPA+ or may have impact on changes in the implementation.