add fontWeight to theme

Follow up issue from #311

@broccolini how does this look for structure? Are there any other font weights we want to include?

fontWeight: {
 bold: '600',
 normal: '400'
}

• Update any instances where fontWeight is hard coded

[broccolini]

I talked with @emplums about this yesterday, I’m not sure we should support semibold in the theme because this should not be used. A few years ago when we introduced system fonts, we reduced the amount of font variations significantly, and made bold lighter and removed semibold instances. I think there are still some instances of semibold but it’s not a weight we should promote.

I agree that we should let people use number values too so they could still override, but with props I’d prefer to only have strings for the ones we want to promote.

Related, we only support font-weight: 300 for font sizes 20px and above because light weight fonts reduce readability. It would be great if we can warn folks on this type of thing if possible.

[shawnbot]

@colebemis not a dumb question! They can be numbers, and I think that the values in the fontWeights theme key should be:

{
  fontWeights: {
    light: 300,
    normal: 400,
    semibold: 500,
    bold: 600
  }
}