<p>
When we use the default master, the style sequence is as follow.
</p>
<pre data-sub="prettyprint:_">
<!-- editable, when edit it will unghost to folder _style/core.css -->
<link href="/_layouts/1033/styles/core.css?rev=5msmprmeONfN6lJ3wtbAlA%3D%3D" type="text/css" rel="stylesheet">
</link>
<!-- optional, if we specify alternate css -->
<link href="/Documents/fred.css" type="text/css" rel="stylesheet">
</link>
<!-- optional, if we specify themes, editable, if edit it will be unghosted -->
<link id="onetidThemeCSS" href="/_themes/Granite/Gran1011-65001.css?rev=12%2E0%2E0%2E4518" type="text/css" rel="stylesheet">
</pre>
<p>
So we can use still use the css selector in the core.css, but we can override the style in core.css, because the alternate style and theme style come after.
</p>
<p>
If we want use customize customized master page, then we need to take control of all the style.
</p>