HTML demos: Support dynamic color values (e.g., #hex colors) via URL parameters

Currently, HTML demos support dynamic text variables (like {{name}}, {{role}}, {{company}}) and image variables through URL parameters or personalized links.

However, it doesn’t seem like color variables are supported. This prevents dynamically changing UI elements like button colors or page backgrounds through URL parameters.

Request: extend the variable system to allow CSS color values (e.g., #hex) to be passed via URL parameters and applied to HTML elements in HTML demos (ie, v_color=

D81616 → #D81616).