You can use a bootstrap theme with SpreadJS. This requires that a style link is added to the head section of the aspx page after the SpreadJS style link. For example, if you have bootstrap.css and bootstrap-theme.css in your project in the css folder, you can add the following style links.

<link href=”css/gc.spread.sheets.x.xx.xxxxx.x.css” rel=”stylesheet” type=”text/css” />

<link rel="stylesheet" href="css/bootstrap.css" />
<link rel="stylesheet" href="css/bootstrap-theme.css" />