dsh-themes: Apply a complete theme to DSH runtime

# Introduction When using DSH (DeepSeek Harness) for daily development, the web interface is the part you stare at the most. It defaults to only one look. If you want to switch to dark mode, follow the system theme, or migrate your VS Code theme that you've used for years, you usually have to manually edit styles, which are lost after a restart. The philosophy of DSH is "everything is a plugin," so appearance customization can also be delegated to plugins. The dsh-themes plugin introduced below is just such a solution: it features a built-in color palette, light/dark/system-following modes, Open VSX search and installation, VS Code theme import, a persistent theme library, and restoration after restart. # What is dsh-themes? dsh-themes is an appearance and theme plugin for the DSH runtime, created by Mang...