dsh-custom-css: A CSS Editor in the Settings Row

`dsh-custom-css` is a UI customization plugin for DeepSeek Harness (DSH) that allows CSS to be edited directly in the settings row, with style changes taking effect instantly. It mimics browser DevTools, offering features such as syntax highlighting, autocomplete, format validation, and element picking. Style files are synced bidirectionally with disk, and the plugin supports multi-file management, version history rollback, and adaptive light/dark themes. By combining local development with runtime debugging, it enables developers to customize the DSH appearance efficiently.