[Dillo-dev] [patch]: fix the default <th> style to match <td>

I had assumed that the <th> style would be the same as the <td> style but with bold and centering added, so I was surprised to see that it's not. The <td> style has some border styling and padding that the <th> style does not, which seems wrong to me. I'm attaching a patch that brings the <th> style in line with my expectations, plus a test file and before/after screenshots. Please comment! Regards, Jeremy Henty