Bug 1847784: Provide a Theme to FedCM Provider Dialog

[titooan]

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[Amejia481]

I was wondering if we could have a generic theme that could be used for other AC components that want to use compose, similar to what we have on FirefoxColors, we don't need something that detailed but something generic enough that could be used in other components. What do you think?

[Amejia481]

Thanks for putting the PR!
This looks good to me!