Support (theme) => value function in theme specification

[benface]

Is your feature request related to a problem? Please describe.
I was expecting the following to work in my theme object:

borders: {
  text: (theme) => '1px solid ${theme.colors.text}',
},

Unfortunately, it doesn’t. border: 'text' doesn’t render anything.

Describe the solution you'd like
It would be nice if that was supported.

Describe alternatives you've considered
I can hardcode the value like this:

borders: {
  text: '1px solid black',
},

But in this case, the text color changes depending on the color mode, so instead of border: 'text' in my sx prop, I have no choice but to do border: (theme) => '1px solid ${theme.colors.text}' every time I want such a border. Not ideal.

Additional context
Thank you so much!

[hasparus]

As a workaround, you can do the following if you didn't turn off useCustomProperties:

borders: {
  text: '1px solid var(--theme-ui-colors-primary)'
}

[benface]

Oh @hasparus 🤦‍♂️

That works perfectly, thank you!