feat(css): 优化主题切换图标显示逻辑

- 调整图标显示规则，确保日夜模式下图标正确隐藏
- 移除重复的图标隐藏样式定义
- 删除已弃用的MBTI标签渐变色样式
- 保持样式代码结构清晰一致

js/about.js

@@ -182,7 +182,7 @@ class ThemeManager {
 
     init() {
         let theme = getStoredTheme();
-        if (theme === 'night') this.root.setAttribute('data-theme', 'night');
+        if (theme) this.root.setAttribute('data-theme', theme);
         $('#theme-btn').toggleClass('is-active', theme === 'night');
         const langForTitle = getStoredLanguage();
         const titleText = theme === 'night' ? (langForTitle === 'zh' ? '白天模式' : 'Day') : (langForTitle === 'zh' ? '黑夜模式' : 'Night');
@@ -191,7 +191,7 @@ class ThemeManager {
         $('#theme-btn').on('click', () => {
             const curr = this.root.getAttribute('data-theme');
             const next = curr === 'night' ? 'day' : 'night';
-            if (next === 'night') this.root.setAttribute('data-theme', 'night');
+            if (next) this.root.setAttribute('data-theme', next);
             else this.root.removeAttribute('data-theme');
             setStoredTheme(next)
             $('#theme-btn').toggleClass('is-active', next === 'night');