fix(auth): using theme color instead of hardcoded value

[dario-digregorio]

Description

Instead of using a hardcoded color value, the Theme colors should be used. This is how the component looks right now with dark mode:

As you can see it is not really readable.

Checklist

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Breaking Change

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[dario-digregorio]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

Just did