The following Joomla CSS classes can be referenced in the template_css.css file when creating your Joomla themes to style specific layout, module elements.

The CSS class list references an article published at the Joomla forum

Other useful atricles covering this topic include

Absalom Media Mambo CSS tutorial

Joomla Template Kit Visual CSS Reference 1.0.X

You can also find a list of very useful CSS summary at w3schools

/**com_contact**/
.componentheading
.contentpane
.contentdescription
.sectiontableheader
.category
.small
.contentpane
.contentheading
.contact_email
.inputbox
.button

/**com_content**/
.componentheading
.contentpane
.contentdescription
.inputbox
.sectiontableheader
.sectiontableentry1
.sectiontableentry2
.sectiontablefooter
.blogsection
.contentpaneopen
.article_seperator
.contentheading
.contentpagetitle
.buttonheading
.small
.createdate
.modifydate
.readon
.pagenav_prev
.pagenav_next
.adminform
.button
.text_area
.blog
.blog_more

/**com_login**/
.contentpane
.componentheading
.inputbox
.button

/**com_newsfeeds**/
.componentheading
.contentpane
.contentdescription
.sectiontableheader
.category
.small
.contentheading

/**com_poll**/
.componentheading
.contentpane
.pollstableborder
.sectiontableheader
.smalldark
.button
.inputbox

/**com_registration**/
.componentheading
.contentpane
.inputbox
.button

/**com_search**/
.componentheading
.contentpaneopen
.inputbox
.button
.searchintro
.small
.highlight

/**com_user**/
.componentheading
.inputbox
.button
.row1
.row2

/**com_weblinks**/
.componentheading
.contentpane
.contentdescription
.sectiontableheader
.tabclass1
.tabclass2
.small
.category
.inputbox

/**com_wrapper**/
.contentpane
.componentheading
.wrapper

/**includes/frontend**/
.moduletable
.newsfeed
.module
.message

/**includes\HTML_toolbar.php**/
.toolbar

/**includes\joomla.php**/
.profiler
.item
.small
.back_button
.buttonheading
.tab-page
.tab
.inputbox

/**includes\joomla.xml.php**/
.paramlist
.editlinktip
.text_area
.inputbox

/**includes\pageNavigation.php**/
.inputbox
.pagenav


/**includes\pathway.php**/
.pathway

/**includes\js\dtree\dtree.js**/
.dtree
.dTreeNode
.node
.clip

/**includes\patTemplate\tmpl\forms.html**/
.message
.tooltip
.tab-page
.tab
.expander

/**mambots\content\mosimage.php**/
.mosimage_caption
.mosimage

/**mambots\content\mospaging.php**/
.pagenavcounter
.pagenavbar
.contenttoc
.toclink

/**mambots\content\mosvote.php**/
.content_rating
.content_vote
.button

/**modules\mod_latestnews.php**/
.latestnews

/**modules\mod_login.php**/
.button
.inputbox

/**modules\mod_mostread.php**/
.mostread

/**modules\mod_mostread.php**/
.poll
.pollstableborder
.button

/**modules\mod_mostread.php**/
.syndicate
.syndicate_text

/**modules\mod_search.php**/
.inputbox
.button
.search