[Theme Generator v3] Support custom font family

[mmailaender]

Describe the feature in detail (code, mocks, or screenshots encouraged)

We have one Figma user that uses a custom font family (Aleo).
The current Theme generator supports only predefined font families.

Expected:

Ideally, the user can set a custom font family in the theme generator to sync it easily from there with the Figma plugin to Figma.

Workaround:

1. The user needs to adjust Figma variables (Base Knowledge about Figma Variables needed)
2. The user adjusts the output theme manually.

What type of pull request would this be?

Enhancement

[endigo9740]

This is already one of the specced enhancements planned for the future, so I've assigned this ticket as a sub-task:

• NEXT: New Theme Generator #1142

For now, if users wish to use a custom font they can, they just need to follow the instructions here:

https://next.skeleton.dev/docs/design/themes#custom-fonts

Note that :root won't be required for the upcoming Tailwind v4 update.