DateTimePicker component: Use color function for defining the background

[mmtr]

Description

The last version of the @wordpress/components package provides an CSS file with invalid syntax because there is background property with a wrong value for the DateTimePicker component.

It seems that it was forgotten to include the color function on #7621, as this function will create a new color by adjusting it with the given shade.

This PR adds that function, so we can solve the import problems we have on Calypso using the latest packages.

How has this been tested?

I tested locally by generating a build and checking that the built CSS include a valid background value.

Testing instructions

• Build the packages with npm run build:packages
• Check that the generated file packages/components/build-style/style.css doesn't include any shade, specially the background properties between lines 1615 and 1629.
• Check that the generated CSS can be imported using SCSS

Types of changes

Bug fix

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.

[mmtr]

cc @vindl @gziolo

[Copons]

I'm not exactly sure how to test this in Gutenberg (@mmtr feel free to add a Testing Instructions section and maybe some before/after screenshots), but the lack of color function was definitely an error and this PR fixes it. ✨

[mmtr]

@mmtr feel free to add a Testing Instructions section and maybe some before/after screenshots

Sorry about that, @Copons! I just added the instructions below:

Testing instructions

• Build the packages with npm run build:packages
• Check that the generated file packages/components/build-style/style.css doesn't include any shade, specially the background properties between lines 1615 and 1629.
• Check that the generated CSS can be imported using SCSS

I don't think screenshots are needed here since we have not changed visually anything.