Update theme colors and enhance dark mode styling

- Changed the dark theme background color in `app.js` for improved aesthetics.
- Added new CSS variables for dark theme elements in `style.css`, enhancing visual consistency and user experience.
- Updated styling for various components in dark mode, including duty markers and calendar sticky behavior, to align with the new theme parameters.

webapp/app.js

@@ -3,7 +3,7 @@
   const RETRY_DELAY_MS = 800;
   const RETRY_AFTER_ACCESS_DENIED_MS = 1200;
 
-  const THEME_BG = { dark: "#1a1b26", light: "#d5d6db" };
+  const THEME_BG = { dark: "#17212b", light: "#d5d6db" };
 
   function getTheme() {
     if (typeof window === "undefined") return "dark";