update transition block options

[wen-2018]

If this changeset needs to go into the FXC codebase, please add the WMO and FXC label.

One-line summary

This block adds dark-alt as a transition block color option.

Significant changes and points to review

• homepage transition block after donation section should dark color to match background color of section below
• about us page transition block after community section should use dark-alt color

Issue / Bugzilla link

Testing

• get Prod data down to local for dev
• http://localhost:8000/cms-admin/
• use preview function to checkout homepage and about us 2026 page transition blocks