I'm quite interested in frontend technology and enjoy to take a look at announcements like these. But at the same time my knowledge about JavaScript is extremel

I'm quite interested in frontend technology and enjoy to take a look at announcements like these. But at the same time my knowledge about JavaScript is extremely limited. So just out of curiosity: Why is the theme fontSize defined as xs: ['0.75rem', { lineHeight: '1rem' }] rather than something like xs: { size: '0.75rem', lineHeight: '1rem' } ?