Is there a list of WordPress CSS id/classes that I can use to build plugin option screens that look and feel more like a normal WordPress option page? I have found some through serendipity but it would have been nice to have a list.
An example is this class: button-primary That makes a button look like this: 
We're working on updating http://dotorgstyleguide.wordpress.com/ to have more of this information and reflect the style updates from 3.2.
Other than that, I've seen a plugin (that I currently cannot locate) that shows a demo page of sorts that displays the various CSS selectors and what they look like, but I believe it was outdated. Other than that, your best option for now may be to just fire up Firebug/Web Inspector and take a look at wp-admin/css/wp-admin.dev.css.
Install this plugin and you see all elements, class and ids https://github.com/bueltge/WordPress-Admin-Style
*Note: For Frontend side - Here is the list which i have extracted after a lot of R&D in wordpress default css styles. I did my best to research everything and organize everything as accurately as possible. If you see anything missing or incomplete, please write in the comments. I hope it might help you to develop your desired plugins and themes *
/* WP WYSIWYG Editor Styles */
.entry-content img {
margin: 0 0 1.5em 0;
}
.alignleft, img.alignleft {
margin-right: 1.5em;
display: inline;
float: left;
}
.alignright, img.alignright {
margin-left: 1.5em;
display: inline;
float: right;
}
.aligncenter, img.aligncenter {
margin-right: auto;
margin-left: auto;
display: block;
clear: both;
}
.alignnone, img.alignnone {}
.wp-caption {
margin-bottom: 1.5em;
text-align: center;
padding-top: 5px;
}
.wp-caption img {
border: 0 none;
padding: 0;
margin: 0;
}
.wp-caption p.wp-caption-text {
line-height: 1.5;
font-size: 10px;
margin: 0;
}
.wp-smiley {
margin: 0 !important;
max-height: 1em;
}
blockquote.left {
margin-right: 20px;
text-align: right;
margin-left: 0;
width: 33%;
float: left;
}
blockquote.right {
margin-left: 20px;
text-align: left;
margin-right: 0;
width: 33%;
float: right;
}
.gallery dl {}
.gallery dt {}
.gallery dd {}
.gallery dl a {}
.gallery dl img {}
.gallery-caption {}
.size-full {}
.size-large {}
.size-medium {}
.size-thumbnail {}
/* WP CSS - Miscellaneous Styles */
/* category links */
li.categories {}
li.cat-item {}
li.cat-item-{id} {}
li.current-cat {}
li.current-cat-parent {}
ul.children {}
/* blogroll links */
.linkcat {}
.blogroll {}
/* read-more links */
.more-link {}
/* WP CSS - Page Listings */
.pagenav {} /* outermost list item */
.page_item {} /* any page item */
.page-item-{id} {} /* specific page id */
.current_page_item {} /* current page */
.current_page_parent {} /* parent of current page */
.current_page_ancestor {} /* any ancestor of current page */
.pagenav ul,
.pagenav .current_page_item ul,
.pagenav .current_page_ancestor ul,
.pagenav .current_page_ancestor .current_page_item ul,
.pagenav .current_page_ancestor .current_page_ancestor ul,
.pagenav .current_page_ancestor .current_page_ancestor .current_page_item ul,
.pagenav .current_page_ancestor .current_page_ancestor .current_page_ancestor ul {}
.pagenav ul ul,
.pagenav .current_page_item ul ul,
.pagenav .current_page_ancestor ul ul,
.pagenav .current_page_ancestor .current_page_item ul ul,
.pagenav .current_page_ancestor .current_page_ancestor ul ul {}
.pagenav .current_page_ancestor .current_page_ancestor .current_page_item ul ul,
.pagenav .current_page_ancestor .current_page_ancestor .current_page_ancestor ul ul {}
/* WP CSS - Default WordPress Widgets */
.widget {}
/* links widget */
.widget_links {}
.widget_links ul {}
.widget_links ul li {}
.widget_links ul li a {}
/* meta widget */
.widget_meta {}
.widget_meta ul {}
.widget_meta ul li {}
.widget_meta ul li a {}
/* pages widget */
.widget_pages {}
.widget_pages ul {}
.widget_pages ul li {}
.widget_pages ul li a {}
/* recent-posts widget */
.widget_recent_entries {}
.widget_recent_entries ul {}
.widget_recent_entries ul li {}
.widget_recent_entries ul li a {}
/* archives widget */
.widget_archive {}
.widget_archive ul {}
.widget_archive ul li {}
.widget_archive ul li a {}
.widget_archive select {}
.widget_archive option {}
/* tag-cloud widget */
.widget_links {}
.widget_links li:after {}
.widget_links li:before {}
.widget_tag_cloud {}
.widget_tag_cloud a {}
.widget_tag_cloud a:after {}
.widget_tag_cloud a:before {}
/* calendar widget */
.widget_calendar {}
#calendar_wrap {}
#calendar_wrap th {}
#calendar_wrap td {}
#wp-calendar tr td {}
#wp-calendar caption {}
#wp-calendar a {}
#wp-calendar #today {}
#wp-calendar #prev {}
#wp-calendar #next {}
#wp-calendar #next a {}
#wp-calendar #prev a {}
/* category widget */
.widget_categories {}
.widget_categories ul {}
.widget_categories ul li {}
.widget_categories ul ul.children {}
.widget_categories a {}
.widget_categories select{}
.widget_categories select#cat {}
.widget_categories select.postform {}
.widget_categories option {}
.widget_categories .level-0 {}
.widget_categories .level-1 {}
.widget_categories .level-2 {}
.widget_categories .level-3 {}
/* recent-comments widget */
.recentcomments {}
#recentcomments {}
#recentcomments li {}
#recentcomments li a {}
.widget_recent_comments {}
/* search widget */
#searchform {}
.widget_search {}
.screen-reader-text {}
/* text widget */
.textwidget {}
.widget_text {}
.textwidget p {}
/* WP CSS - Comment Styles */
.commentlist .reply {}
.commentlist .reply a {}
.commentlist .alt {}
.commentlist .odd {}
.commentlist .even {}
.commentlist .thread-alt {}
.commentlist .thread-odd {}
.commentlist .thread-even {}
.commentlist li ul.children .alt {}
.commentlist li ul.children .odd {}
.commentlist li ul.children .even {}
.commentlist .vcard {}
.commentlist .vcard cite.fn {}
.commentlist .vcard span.says {}
.commentlist .vcard img.photo {}
.commentlist .vcard img.avatar {}
.commentlist .vcard cite.fn a.url {}
.commentlist .comment-meta {}
.commentlist .comment-meta a {}
.commentlist .commentmetadata {}
.commentlist .commentmetadata a {}
.commentlist .parent {}
.commentlist .comment {}
.commentlist .children {}
.commentlist .pingback {}
.commentlist .bypostauthor {}
.commentlist .comment-author {}
.commentlist .comment-author-admin {}
.commentlist {}
.commentlist li {}
.commentlist li p {}
.commentlist li ul {}
.commentlist li ul.children li {}
.commentlist li ul.children li.alt {}
.commentlist li ul.children li.byuser {}
.commentlist li ul.children li.comment {}
.commentlist li ul.children li.depth-{id} {}
.commentlist li ul.children li.bypostauthor {}
.commentlist li ul.children li.comment-author-admin {}
#cancel-comment-reply {}
#cancel-comment-reply a {}
/* WP CSS - body_class() */
.rtl {}
.home {}
.blog {}
.archive {}
.date {}
.search {}
.paged {}
.attachment {}
.error404 {}
.single postid-(id) {}
.attachmentid-(id) {}
.attachment-(mime-type) {}
.author {}
.author-(user_nicename) {}
.category {}
.category-(slug) {}
.tag {}
.tag-(slug) {}
.page-parent {}
.page-child parent-pageid-(id) {}
.page-template page-template-(template file name) {}
.search-results {}
.search-no-results {}
.logged-in {}
.paged-(page number) {}
.single-paged-(page number) {}
.page-paged-(page number) {}
.category-paged-(page number) {}
.tag-paged-(page number) {}
.date-paged-(page number) {}
.author-paged-(page number) {}
.search-paged-(page number) {}
/* WP CSS - post_class() */
.post-id {}
.post {}
.page {}
.attachment {}
.sticky {}
.hentry {}
.category-misc {}
.category-example {}
.tag-news {}
.tag-wordpress {}
.tag-markup {}
The WordPress backend does have generated "body" classes. I have never seen these explicitly documented anywhere (though I could well have missed it). You can see them and how they are generated in the admin-header.php source.
The generated classes are very similar to the front-end ones:
<body class="wp-admin no-js upload-php admin-bar branch-3-3 version-3-3-1 admin-color-fresh">
I got to this question because I was creating a custom admin page using add_menu_page(). You're going to want to put your content between
<div class = "wrap">Your content.</div>
This will activate the standard WordPress admin backend CSS so that your custom admin page looks normal. Other divs would be automatically handled by WordPress in this case.
class AdminScreen{
public function __construct(){
add_action( 'admin_menu', array( $this, 'doAddMenuPage' ) );
}
public function doAddMenuPage(){
add_menu_page( "AdminScreen", "AdminScreen", 'edit_others_pages', 'AdminScreen', array($this, 'echoAdminPage') );
}
public function echoAdminPage(){
//Prints out the HTML into the output buffer:
echo '<div class = "wrap"><h1 class = "wp-heading-inline">This looks good, finally!</h1></div>';
}
}
Here is an other interesting article that makes a decent job of listing all the available css classes/html markup one can use in styling an admin page. The articles is a little outdated in that it show styling results from older version of WordPress, so its likely that additional classes have been added since.
Another useful resources are the WordPress dashicons.
However, I have to congratulate @bueltge on his excellent plugin listed below which does a great job of keeping an updated reference of admin styles.
