Following Styles in SharePoint Customization
When customizing or writing add-ins for SharePoint it’s important to follow the styles such as ms-input or ms-descriptiontext on your components so that your pages will use modified CSS files, master pages etc.
You can download and install the Microsoft Developer Toolbar for IE 6 or 7 and click on the IE Developer Toolbar button. This displays the Developer Toolbar pane. Click the “mouse pointer” icon on the left side of the bar, select an item on the page and information about the HTML element will be displayed: