Newsletter Email templates: support custom colours

We should allow customizing colours in the email template, or ideally pull colours from the theme.

Project thread: peKye1-13y-p2

Our default template colours are blue (WP.com) and green (Jetpack) — applied to CTAs and links:

When using [_Lettre_ theme](https://wordpress.com/theme/lettre), we apply colours to templates (And subscription modal!) directly from the theme styles;

This doesn't work universally because different themes declare different "primary colour" variables. We also have the challenge of knowing if the colour works on a white background with white text.

Start with the Newsletter email templates and the subscription modal, using the Lettre theme styles described in the issue as the reference case. Compare the default blue and green CTA/link colours with theme-provided colours, then define a solution that handles differing primary-colour variables and contrast on white backgrounds.

Written by the indexing model from the issue text.