Edit your masterpage file, In the tag add a link to your custom CSS file just above the content place holder named “PlaceHolderAdditonalPageHead” ie.

<link href="/Style%20Library/mystyle.css" rel="stylesheet" type="text/css" />

Sample CSS to customise the ‘Site Actions’ buttons in Sharepoint 2010

/* Site Actions Menu */
.ms-siteactionsmenu{
border:1px solid transparent;
background-color:#F2F2F4;
}
.ms-siteactionsmenu .ms-menu-a span{
color:#000000;
}
.ms-siteactionsmenu .ms-siteactionsmenuhover {
background-color:#FFFFFF;
color:#ffffff;
}

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To style the text of the webpart header you use the .ms-WPTitle class.

Some webarts automatically make the title of the webpart ie. a link to the specific list its pulling data from.

To style these headers use something like:

/* Webpart Header */
.ms-WPHeader {
	BACKGROUND-COLOR: #EEEEEE;
}
.ms-WPHeader td {
	border-bottom:1px solid #0066cc;
}
.ms-WPTitle,
.ms-WPTitle a:link,
.ms-WPTitle a:visited {
color:#990066;
}

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When you apply a custom master page you may find that it knocks out the positioning of your dialog boxes & you can no longer see the OK & Cancel buttons beacuse they have been knocked down the page. This is because now your new masterpage shiny banner (in my case) is also displaying in the dialog box. To stop this happening is nice & easy thanks to Microsoft:

Simply add ‘s4-notdlg’ to the class ie.

<div class="customHeader">
becomes

<div class="customHeader s4-notdlg">

Sharepoint 2010 Base CSS Classes

http://erikswenson.blogspot.com/2010/01/sharepoint-2010-base-css-classes.html