Discuss: upgrade default styles to use flexbox

[vibegui]

I noticed that there's a fair amount of arbitrary margins going on to "center" content. This also makes theming harder, as one needs to override all these margins. I think we could embrace flexbox and provide "simpler" out-of-the-box styling.

What do you think? @jdan

[jdan]

I think it's a great idea. I tend to make slide-decks with vertically-centered text, so I would be game for it.