Customizable font styles and colors for changelog content

Description: Enable customization of font style (family, size, weight, italics) and text color for changelog content, at global, per-entry, and per-block levels in the editor and via API. Problem: Current changelog styling is fixed, preventing teams from aligning updates with brand guidelines, establishing visual hierarchy, and ensuring accessible contrast. This limits readability, reduces engagement, and creates inconsistency between web embeds and in-app views. Benefit: Maintain brand consistency across all changelog surfaces (web, embed, in-app). Improve readability and accessibility through adjustable typography and color contrast. Increase engagement and comprehension by visually emphasizing critical updates (e.g., breaking changes, deprecations). Ensure parity between UI editor and API-driven content, enabling automated yet branded publishing.