Playground CSS improvements

[thangngoc89]

This PR addresses #105 in attempt to make the playground looks nicer.
Things done:

• Vertically aligned headers and items inside it
• Change Run button color + add border radius

All numbers are based on scale of 4px.

A picture is better than a thousand words so here we go:

[aantron]

Thanks! Thsi branch is live now. Except for the thing with the link, it's ready to merge.

[thangngoc89]

@aantron I updated that. It looks just like before :

[aantron]

It looks misaligned now that everything is aligned :) Or would you prefer to keep it that way in this PR?

[aantron]

Alright, so it's actually aligned with the button, but because of the button colors, the eye wants the baseline of the link to be aligned with the baseline of the title. That can just be "fixed" later or ignored. Thanks!

[thangngoc89]

Run and Docs for this example are actually aligned but it looks misaligned because of the color illusion. I think we would need to design this in a design program to see how things go. It's easier to move things around by drag and drop then write css code.

[aantron]

It's now live and already looks considerably better. I guess even the button alone is like 10% of the UI or so.