Global formatting properties in Report composer

in xml templates of the report composer I can refer to styles which can be customized by the users. When I try to change the background colour of the table headings only the header text gets a different background, but not the whole line.
I says after generation of the html documentation that 2 different CSS files are generatied. One is containing the report specific definition the ofther the global setttings ( e.g. table layout ) .
Can this global layout also be customized ?
If yes will it that be appliable to all output formats (HTML, PDF, WORD) ?

Kind Regards