Goal:

Allow customers to customize more theme options (or override the theme "completely").

This might contain other smaller continuous improvements/follow-ups with the theme.

Background:

Follow up on Feature 1.2 RHIDP-281 (RHDH Theme for Backstage) and Epic RHIDP-1160 (Create custom theme for Backstage), which mostly externalize the theme into its own npm package and align the current theme with PatternFly.

Why is this important?

Customers and partners want more control over the theme / look and feel of their RHDH instance.

Acceptance criteria:

1. app-config.yaml options to customize and override all major Backstage theme colors and fonts.
2. app-config.yaml options to customize and override our own component defaults.
   1. app-config.yaml options to re-enable the rippleEffect for example
3. Provide TypeScript types, defaults, and documentation (in the theme repo first, later in the official documentation?) on which attributes are available.