chore(sfc-playground): style improvements

[phanan]

This PR adds some (hopefully) improvements to the current style of the playground, namely:

• Standardize the buttons to use flexbox instead of middle-aligning and/or line-height for vertical/horizontal centering
• Update the SVGs to use currentColor instead of arbitrary color values, allowing controlling their colors from CSS, which in turn allows hover effects
• Make prod/dev env badge more prominent
• Some minor improvements/fixes for CSS rules and variables

Before

After