All you need is a plain text editor (I personally recommend Notepad++) and edit the appropriate file. If you want to edit the header that displays on every page you can edit styles/style-name-here/template/overall_header.html
We would love to help but we need more information. Did you get an error message? When did this happen? Were you updating or installing a MOD? Please elaborate and fill this out: Support Request Template. Thanks.
Hi and welcome. :) phpBB's proSilver and subSilver2 styles are already web standards compliant. And you can easily edit the CSS and XHTML to change it as you like. You just need to edit the appropriate file in the styles/prosilver directory. And phpBB doesn't have friendly URL support out of the box...
A blank page means that there's an error somewhere but error messages have been turned off by your host. Can you try making a new topic again then immediately check the server error log? (Ask your host if you don't know where it is.) Then post the last 20 or so lines of it for us to look at. Thanks!
Can you check your permissions and see if you can create new topics? Go to the ACP -> Permissions -> Forum permissions -> select forum(s) -> select group(s) and click Edit permissions -> select the Post tab and make sure that "Can start new topics" is set to Yes.
You don't need to do that because Google is looking for real content not keywords. However if you still want to, you would need to edit styles/prosilver/template/overall_header.tpl with a plain text editor like Notepad.