Add some dark theme tweaks and small html fix

@peterflynn I am not sure if I chose colors that suit you, but I did make it so that the matches and matched groups show up ok on a dark theme.

When I was looking at the styles I noticed that the resulting HTML when you did `.show` on `inline-regex-groups` ended with an orphaned `style>` so I just added `.css()` to explicitly change the style to `display: inline` to avoid the orphaned tag.  Not a big deal since it worked fine before ;)

Great extension, btw!