dsh-plugin-custom-css: Custom CSS Injection for DeepSeek Harness

`dsh-plugin-custom-css` is a plugin for DeepSeek Harness (DSH) Desktop that allows users to toggle and inject custom CSS in real time from the settings panel to customize the interface style. The plugin supports Harness design tokens (such as CSS variables), stores configuration locally, and does not modify the Harness installation directory. Installation requires adding it in the Web configuration directory using pnpm and registering it in `package.json`; after restarting the application, users can edit code through the settings panel to adjust themes.