Thanks for your reply ... yes that is a way but not very clean. 

I wanted to know where the web.report_assets_common.0.css was located

Hi,

You can activate the developer mode and from Settings -> Technical -> Actions -> Report, search the report and open it, then click the Qweb Views smart button in the top right and open the record named report_saleorder_document , here you can see how the report is defined and make the necessary changes from here. Either you can remove the h2 class or define the style for the corresponding span.

Thanks