Add some dark theme tweaks and small html fix

[mackenza]

@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!

[mackenza]

e8353f5 was added because I noticed that if you have a font size (possible now that we have themes) other than the default 12px, the overlay for the sample match doesn't line up properly.

I suppose this fix might not be satisfactory, it fixes the sample text to be always 12px regardless of theme setting, it seemed easier than doing math to figure out how many pixels wide each of the various font sizes are before rendering the overlay. Your call ;)