ui-song-theme: Fallback DeepSeek Harness Song-inspired Light/Dark Theme

`ui-song-theme` is a web interface theme plugin for DeepSeek Harness (DSH) developed by yunfei07 and licensed under the MIT license. This plugin aims to provide a set of light and dark visual tokens based on a Song-inspired style for DSH. It addresses the needs of users who wish to adjust the interface appearance while preserving their existing light, dark, or system preferences, and require the ability to fully restore the initial state upon uninstallation. Its core mechanism involves using `ThemeRuntime.overrideTokens()` to overlay a reversible theme layer rather than directly modifying the base theme, and replacing traditional blue selection boxes with a unified plant color family. Installing this plugin requires Node.js ^22.19.0 or >=24.0.0.