Fix @theme-ui/color types

[allanpope]

@hasparus Thanks for picking up that the parameter order was wrong, totally missed that 🤦‍♂.
Have fixed up the parameter order, added more tests and marked number in the mix function as optional, as the source has a default param.

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[typescript-bot]

@allanpope Thank you for submitting this PR!

Because this PR doesn't have any code reviewers, a DefinitelyTyped maintainer will be reviewing it in the next few days once the Travis CI build passes.

In the meantime, if the build fails or a merge conflict occurs, I'll let you know. Have a nice day!

[hasparus]

TBH, all of these functions are 2nd order and take theme parameter at the second invocation. I'm also not sure if the tests are right https://theme-ui.com/packages/color/. Instead of #ff0 we receive primary, a key of theme.colors.

Take a look at .d.ts emitted in system-ui/theme-ui#672.

[allanpope]

@hasparus Yeah, so it takes a theme color eg: 'primary' as the first parameter. Within the function it does a get to pull the value out of the theme object.

Are you suggesting that I pull out the theme color names from the theme object and then use them? eg:

// Ideally grab these from theme object
const themeColors = 'primary' | 'secondary' etc..

export function darken(color: themeColors, number: number | string): string;

I didn't know that you had opened a PR on theme-ui for this. Since theme-ui will have built in types, I may as well close this PR, use yours once it's merged in and remove this package from DT.

[typescript-bot]

@allanpope The Travis CI build failed! Please review the logs for more information.

Once you've pushed the fixes, the build will automatically re-run. Thanks!

[PranavSenthilnathan]

I'll close this for now as mentioned in the comments. If you wanted to have this merged, feel free to reopen or send another PR.