By default the widgets in a bonitasoft form have no borders. You need to change the css / less files of the bonitasoft theme that is used. In the commercial editions that can be done by creating a new look 'n' feel, as explained in this article:
http://documentation.bonitasoft.com/creating-new-look-feel
However, this functionality is not available in the community edition. In the community edition it can be done, you have to edit the files under:
<BONITA_HOME>\client\tenants\1\work\forms\<Processname>\<ProcesInstanceId>\resources\application\css\bonita_form_default.css
You need to edit the
.bonita_table_container
class and set some border properties with css.
Be carefull, a new processinstance (with a new id and directory) will be generated every time you redeploy.