Adding meta tag keywords - the same or different on all pages?

You should use separate meta tags for each page of your site and the content of these tags should reflect the page content rather than the site theme.

For the record you are only going to get so far with Meta tags.

By far the most important thing to consider is the text within your title tag.

The Meta description tag can be useful and will be used by Google et al to create a search snippet (though it won't always be used and Google will grab bits of content relevant to the user's search query).

The Meta keywords tag is more or less useless. It won't do any harm to include it but don't expect it to have any effect at all.

Remember: Search engines index PAGES and not SITES. Make the content, title, meta description, any text linking TO the page and any ad campaigns pointing at the page all tie in together using your targeted key words and phrases.

The single most important thing is your page content. Make it "good", and don't try to build a site for a search engine - instead, build it for people. That is how to get good search visibility.