To use the plugin you can download it or use as Bower dependency:
"ui-grid-bootstrap-styles"
As soon as you've got all the files downloaded and included in your page you just need to declare a dependency on the ui.grid.bootstrap.style module:
angular.module('myModule', ['ui.grid.bootstrap.style']);
Example:
<div class="grid" ui-grid-bootstrap-styles ui-grid="gridOptions"></div>