The easiest way to change the look and feel of the grid is to use on of the predefined themes. For a server-control implementation implementation set the Theme property. For a client-side implementation pass the theme name as an argument to the handler that delivers the DbNetSuite CSS. e.g.
<link rel="stylesheet" type="text/css" href="dbnetsuite.css.ashx?theme=light" />
You can run the themes sample to see a theme applied to DbNetGrid.
Sample Themes
Classic
Light
Dark
You can customise the look and feel of the DbNetSuite components more selectively by adding CSS that either compliments or overrides the default CSS that is emitted by the DbNetSuite components.
You can see examples of customised component styles for DbNetGrid, DbNetEdit and DbNetFile
You can see a full listing of the default CSS styles emitted by DbNetSuite here.