style(theme): expand color palettes and refine dark mode styles

- Expand CSS variables for primary and secondary colors to include full 50-950 scales in `main.css`.
- Update components to reference specific color shades (e.g., `primary-400`, `secondary-200`) instead of generic variables.
- Add dark mode background and text colors to Events, Hall of Fame, and Index sections.
- Adjust image aspect ratio in the Events component.

app/layouts/default.vue

@@ -12,7 +12,7 @@
           <div class="flex gap-4 items-center">
             <img class="inline h-12 w-auto" src="/Logo.svg" alt="YPHS Alumni" />
             <h1
-              class="text-xl font-bold text-gray-900 hover:text-primary hidden md:inline"
+              class="text-xl font-bold text-gray-900 hover:text-primary-400 hidden md:inline"
             >
               永平中学校友会
             </h1>