Flag for non-theme namespace colors

[taysea]

Please describe what the rule should do:

This rule should flag when non-theme colors are used.

What category of rule is this? (place an "X" next to just one item)

[ ] Warns about a potential error (problem)
[x] Suggests an alternate way of doing something (suggestion)
[ ] Other (please specify:)

Provide 2-3 code examples that this rule will warn about:

<Text color="#453DF3" />
<Text color="skyblue" /> (where skyblue doesn't exist in the user's theme)

Why should this rule be included in eslint-plugin-grommet?

Best practice for consistency across an app to use the theme as the color namespace.