fix(a11y): text color contrast

[mfranzke]

Issue Summary

The signing up terms textual paragraph get's an 80% opacity defined on BookEventForm, that for this text colors contrast violates Success Critera 1.4.3 that expects a color contrast of at least 4.5:1

Steps to Reproduce

1. Go to a book event form.
2. Select a time. You should get to confirmation page on which you are requested to provide your name and email, compare to the following screenshot:

Actual Results

On the confirmation page where you enter your name, you'll recognize that the signing in terms text has a color contrast which is too low.

Expected Results

The text should be accessible.

Technical details

independent from browser environment.

Evidence

Run Google Lighthouse out of Google Web Developer tools to check for any reportings on this topic.