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.

content/news/20251128-misunderstanding-yphsalumni-and-alumnigroup.md

@@ -4,13 +4,13 @@ date: "2025-11-28"
 updated: "2025-11-28"
 author: "永平中学校友会"
 description: "毕业校友常说“我都加入校友群组了”，实则误入了校友联络处。本文厘清其与永平中学校友会的区别与使命。"
-cover: "https://img.tootaio.com/i/2025/11/28/ex7pen.jpeg"
+cover: "https://img.yphsalumni.org/i/2025/11/28/sbk8ue.jpeg"
 tags: ["永平中学", "校友会", "校友联络处", "区别", "公告"]
 category: "公告"
 highlight: true
 seoTitle: "永平中学校友会与校友联络处的区别 | 官方说明"
 seoDescription: "毕业校友常说“我都加入校友群组了”，实则误入了校友联络处。本文厘清其与永平中学校友会的区别与使命。"
-ogImage: "https://img.tootaio.com/i/2025/11/28/ex7pen.jpeg"
+ogImage: "https://img.yphsalumni.org/i/2025/11/28/sbk8ue.jpeg"
 ---
 
 **永平中学校友会致广大校友的一封信**