[core-v2.4.0-alpha.8] : Enhancement - Add --onboard-modal-color

[ReflectiveChimp]

Description

Adds --onboard-modal-color (to compliment --onboard-modal-background).

Maintains the current default color initial (via all: initial on <onboard-v2/>)

Needed to change default text color for modals such as:

Checklist

• The version field in package.json is incremented following semantic versioning
• The box that allows repo maintainers to update this PR is checked
• I tested locally to make sure this feature/fix works
• This PR passes the Circle CI checks

[Adamj1232]

@ReflectiveChimp looks good, thank you for the PR! Could you also add the variable to this section Probably needs a new group

/* CUSTOMIZE THE SHARED MODAL */
--onboard-modal-background
--onboard-modal-color

I will get this merged today